Logitech G9
After the brief adjustment time from my old MX510, I have come to really like it. The mouse itself is great and comfortable with the small grip. The software is how you switch "modes" is great too. On my mouse now I have one regular option, one where the side mouse buttons are keyboard keys (for games that don't support side mouse buttons), and one with no acceleration. That, in addition to speed switching, makes almost any game I play feel comfortable. It's even great for things like Wolfenstein (the new one) or Vice City PC where the Y-axis speed is different than X; it is easy to alt tab out of the game and drop/raise it as I need to.
I'd still like to see Logitech advance this and the MX510 style. I know they released the G500 in that style a while back. One thing I liked about that was that it seemed to have three side mouse buttons.
I never understood why two seemed to be the limit for side buttons on "gaming" mice. I've seen all purpose mice with more, console mice solutions with four (to match PS3/360 face buttons), and that Razer mouse with a number pad. Since I play PC and console games a lot, I'm used to letting my right thumb getting more action. I'm sure they can work out a layout that works.
edit: see that Logitech Performance Mouse MX posted above. Four side buttons on there.