X-Mouse Button Control 2.6 + Portable 
 4.1/5.1 Mb

X-Mouse Button Control is a windows application to remap your mouse buttons. 
You are able to provide an application specific mappings, which means one application can use the mouse differently from another. 
This is useful for games which do not inherently support the extended mouse buttons, because you can map keys to each button.

What's more, the list of functions available to map to is somewhat more comprehensive than the 32bit MS Intellimouse Software can handle, including things like:

Copy/Cut/Paste
Volume Up/Down/Mute
Media Player control
Send a custom keystroke sequence
Launch your email (or any other) application.
Capture screen (or active window) image to clipboard.
Click-Drag [Sticky Buttons].
Save and restore desktop icon positions.
Vista (and Windows 7) support including some Vista/7 only featurs such as 'Flip 3D'.