I've got a mouse with a thumb button that xev shows as button 9. I'd like to map it to be the same as a middle click (which, according to xev, is button 6). How do I do it? I'm using Ubuntu Jaunty.
I think you'll be able to get this working with an xmodmap tweak. Have a look at the Mouse section in the Ubuntu Community documentation, especially the entries for the Multi-buttoned Mouse and Logitech Marble Mouse USB.
You'll also find this article on changing mouse buttons through xmodmap useful.
There are a ton of tutorials for this out on there on the internet.
This one is Ubuntu specific : https://help.ubuntu.com/community/ManyButtonsMouseHowto