From Gentoo Linux Wiki

[edit] XOrg 6.8 or Lower and/or PS/2 Mice

[edit] Logitech MX310

If you have a Logitech MX310, change the xorg.conf file to read

...
Section "InputDevice"
   Identifier  "Mouse1"
   Driver      "mouse"
   Option      "Protocol"     "ExplorerPS/2"
   Option      "Device"       "/dev/input/mice"
   Option      "Buttons"      "7"
   Option      "Emulate3Buttons" "false"
   Option      "ZAxisMapping" "4 5"
   Option      "ButtonMapping" "1 2 3 6 7"
EndSection
...

Note: The window-switch button below the scroll wheel is not included. It isn't recognized without using the evdev driver.

[edit] Logitech MX400 and MX615

without tilt wheel

...
Section "InputDevice"
   Identifier  "Mouse1"
   Driver      "mouse"
   Option      "Protocol"     "ExplorerPS/2"
   Option      "Device"       "/dev/input/mice"
   Option      "Buttons"      "7"
   Option      "ZAxisMapping" "4 5"
   Option      "Emulate3Buttons" "false"
   Option      "ButtonMapping" "1 2 3 6 7 8"
EndSection
...

[edit] Logitech MX500

# for xorg-x11 7.0-r1, this worked for me (MX500 in PS/2 port)
Section "InputDevice"
       Identifier      "MX500"
       Driver          "evdev"
       Option          "Device"                "/dev/input/event1"
       Option          "ButtonMapping"         "1 3 2 4 5 6 7 8 9 10"
EndSection

or an alternative:

Section "InputDevice"
       Identifier      "mx500"
       Driver  "mouse"
       Option "Protocol"       "ExplorerPS/2"
       Option "Device"         "/dev/input/mice"
       Option "Buttons"        "7"
       Option "ZAxisMapping"   "6 7"
EndSection

 
#xmodmap -e "pointer = 1 2 3 8 9 4 5 6 7 10 11" <--- edit: I only counted 10 buttons on my MX500 ... o.O

note: 
Position - Number -> Button - Action in Firefox etc
1-1 -> Left Button - Select
2-2 -> Wheel/Middle Button - Paste selection
3-3 -> Right Button - Context menu
4 ?
5 ?
6-4 -> Scroll wheel & button up
7-5 -> Scroll wheel & button down
8-6 -> Back Thumb Button - Backward
9-7 -> Front Thumb Button - Forward
10 ?
11 ?

[edit] Logitech MX600

This mouse seems to only work with the first 9 buttons (mouse actually has 12) when plugged into a PS/2 port (I recommend PS/2 if you got the MX600 as part of the MX3000 Cordless desktop kit, otherwise less than half of the extended keyboard keys will work in USB mode due to a bug in the 2.6 series kernel USB keyboard driver). The buttons that do not work are the Zoom-, Zoom+ and 100% buttons. The following is a working X.Org 7.0+ configuration:

Section "InputDevice"
       Identifier "Mouse0"
       Driver "evdev"
       Option "Dev Name" "ImExPS/2 Logitech Explorer Mouse"
       Option "Device" "/dev/input/event2"
       Option "Buttons" "12"
       Option "ZAxisMapping" "4 5"
EndSection

The default mapping looks like this:

Because the history forward and history backward functions will seem backwards to some people, use the following xmodmap command to fix it:

# xmodmap -e "pointer = 1 2 3 4 5 7 6 8 9"

If you are like me and would rather use the thumb side buttons to go forward and backwards, then use the following (the one furthest away from your body will go forward in the history, the one closest will go back, reverse the 6 and 7 below if you want the opposite):

# xmodmap -e "pointer = 1 2 3 4 5 8 9 7 6"

[edit] Logitech MX610

Section "InputDevice"
   Identifier  "Mouse0"
   Driver     "mouse"
   Option     "CorePointer"
   Option     "Emulate3Buttons" "no"
   Option     "Device" "/dev/input/mice"
   Option     "Protocol" "auto"
   Option     "Buttons" "12"
   Option     "ZAxisMapping" "4 5"
   Option     "ButtonMapping" "1 2 3 9 8 6 7 10 11 12"
   Option     "Resolution" "800"
EndSection

Volume+, Volume-, VolumeMute, Email and IM buttons may not work. Email and IM lights require HIDDEV support in the kernel and a userspace utility such as mx610hack.

[edit] Microsoft Intellimouse Explorer 3.0

Microsoft IntelliMouse Explorer 3.0 USB and PS/2 Compatible (I did this w/ the little green USB -> ps/2 adapter)

...
Section "InputDevice"
   Identifier  "Mouse1"
   Driver      "mouse"
   Option "Protocol"       "ExplorerPS/2"
   Option "SendCoreEvents" "true"            # Not sure if this line is needed ??
   Option "Device"         "/dev/input/mice"
   Option "Buttons"        "4 5"             # side buttons scroll up/down
   Option "ZAxisMapping"   "6 7"             # wheel moves forward/back in the browser
EndSection
...

FYI: This was the only way I could manage to get xev to even recognize the 2 side buttons on the mice at the same time that it recognized the wheel movements. Notice that the buttons now scroll the mouse, and the wheel moves forward/back (next/back) in a browser. To fix this I had to remap the mouse buttons by adding the following line to my startup script. (read below on which file this is for you)

xmodmap -e "pointer = 1 2 3 6 7 4 5"

Kaste: What version of Xorg is that? I had those issues with an intellimouse explorer 4.0 as well when using 6.8 (actually they were probably worse because it has two wheel directions) but they went away when using 7.0 and it still works under 7.1

Metatinara: I had to do my mapping a little differently with Xorg 7.1.

xmodmap -e "pointer = 1 2 3 8 9 4 5 6 7"

This resolved a difference I am having with my side buttons being recognized as 8 and 9 instead of 4 and 5. I am using the same xorg.conf as the previous entry and I have my mouse plugged in via USB.

[edit] Logitech Wireless Optical

I found this from the Xorg documentation. It works for Logitech Wireless Optical and makes the thumb button a browser back button and the scrollwheel to work in modular xorg without the need to use xmodmap.--Rakshasa 13:24, 29 March 2006 (GMT)

Section "InputDevice"
       Identifier      "Mouse0"
       Driver          "mouse"
       Option          "CorePointer"
       Option          "Protocol"      "ExplorerPS/2"
       Option          "Device"        "/dev/input/mice"
       Option          "ZAxisMapping"  "4 5"
       Option          "Buttons"       "6"
       Option          "ButtonMapping" "1 2 3 6 4 5"
EndSection

The following is not the correct way to use the button mapping in XOrg 7 according to the Xorg 7.0 documentation. You have X buttons, not including the wheel up/down, only the wheel click. In the case of my IMExplorer, 2 side buttons, left, right, and wheel click... 5 buttons. Set buttons to 5. Set your wheel number with the ZAxis mapping. Then do not use those numbers in the button mapping, only the 5 buttons. This replaces the need for xmodmap. XOrg 7.2 may be different, who knows there's no online XOrg 7.2 documentation. In any case, the following works:

...
Section "InputDevice"
   Identifier  "Configured Mouse"
   Driver      "mouse"
   Option      "CorePointer"
   Option      "Device"          "/dev/psaux"
   Option      "Protocol"        "ExplorerPS/2"
   Option      "Buttons"         "5"
   Option      "ZAxisMapping"    "4 5"
   Option      "ButtonMapping"   "1 2 3 6 7"
EndSection
...

[edit] Logitech MX700

    Option "ZAxisMapping" "4 5"
    Option "ButtonMapping" "1 2 3 8 9 4 5 6 7 10 11"

Putting this in the mouse section with the standard setup after migrating to modular worked for me. Works here too.

[edit] A4Tech with Two Wheels (WOP-35)

I have a A4Tech WOP-35 mouse with two scrolls and two side buttons. No matter what I wrote in xorg.conf I couldn't get it so that the side buttons were assigned numbers 6 and 7. It always gave them 8 and 9 but this is unwanted since browsers like Opera or Firefox use buttons 6 and 7 for navigating back and forward. So you can't avoid the xmodmap thing. I put it into ${kdedir}/share/config/kdm/Xstartup so that all users have the same setup:

xmodmap -e "pointer = 1 2 3 4 5 8 9 6 7 10 11"

...
Section "InputDevice"
   Identifier     "Mouse0"
   Driver         "mouse"
   Option         "Protocol" "ExplorerPS/2"
   Option         "Device" "/dev/input/mice"
   Option         "ZAxisMapping" "4 5"
   Option         "Buttons" "7"
EndSection
...

Now both scroll buttons do scroll the same and side buttons are used for navigation, no more settings required.

[edit] Kubuntu Breezy Users

Under KDE xmodmap is unnecessary. Change your xorg.conf file to match this:

...
Section "InputDevice"
       Identifier      "Configured Mouse"
       Driver          "mouse"
       Option          "CorePointer"
       Option          "Device"                "/dev/input/mice"
       Option          "Protocol"              "ExplorerPS/2"
       Option          "Buttons"               "7"
       Option          "ZAxisMapping"          "4 5"
EndSection
...

Doing so will allow your wheel to still scroll documents / browser windows and your third and fourth mouse buttons (thumb buttons) to perform back / forward in browser windows without the mucking about in startup scripts. It's also portable across all users on a system, which may be important if multiple users share a computer. Finally, it's a cinch to set up / undo / redo since all of the changes are in one place.

I haven't tested this on my Gentoo box - it doesn't run X. But it may provide an easier solution there too.

NOTE: This solution appears near identical to the previous solution, not sure how CorePointer automagically changes the order the buttons are mapped. Take this solution with a grain of salt. If the previous xorg.conf requires xmodmap and this one does not, please post back here.

[edit] Bluetooth Mice

If you have a bluetooth mouse and all these extra buttons are not working, check out the kernel patches at The Official Linux Bluetooth Protocol Website

[edit] Trust GM-4200 Predator

...
Section "InputDevice"
  Driver       "mouse"
  Identifier   "Mouse[1]"
  Option       "Buttons" "7"
  Option       "Device" "/dev/input/mice"
  Option       "Name" "USB Mouse"
  Option       "Protocol" "explorerps/2"
  Option       "Vendor" "Sysp"
  Option       "ZAxisMapping" "6 7"
EndSection
...

 
#xmodmap -e "pointer = 1 2 3 8 9 4 5 6 7 10 11"

note: 
Position - Number -> Button - Action in Firefox etc
1-1 -> Left Button - Select
2-2 -> Wheel/Middle Button - Paste selection
3-3 -> Right Button - Context menu
4 ?
5 ?
6-4 -> Scroll wheel & button up
7-5 -> Scroll wheel & button down
8-6 -> Back Thumb Button - Backward
9-7 -> Front Thumb Button - Forward
10 ?
11 ?

[edit] Trust DS-3200 Wireless Optical

...
Section "InputDevice"
  Identifier   "Mouse1"
  Driver       "mouse"
  Option       "Name"  "MOSART Semi. Wireless Keyboard & Mouse"
  Option       "Buttons" "7"
  Option       "Device" "/dev/input/mice"
  Option       "Protocol" "explorerps/2"
  Option       "Vendor" "Sysp"
  Option       "ZAxisMapping" "6 7"
EndSection
...

 
#xmodmap -e "pointer = 1 2 3 6 7 8 9 4 5 10 11"