I know, there are dozens of threads to this extent all over the place, but I have a very specific need. I want my mouse to have at least one or two buttons which can be set as modifier keys. (Another alt, shift, ctrl, etc). I am currently using a razor lachesis. I'd VASTLY prefer the mouse be wired. I am often on my computer for 12 hours straight on some work days and I'd rather not being going through 6 bucks in double a's a week. If it's not wired, it needs to be rechargeable at very least. My biggest priority however is being able to use one of the mouse buttons as a modifier.
my guess would be that you want drivers/software capable of this. the actual mouse doesnt really matter.
i know my old logitech mx1000 mouse had custom keymapping in the software. i used to put letter keys (and even shift) as buttons. perhaps check into their gaming mice versions
the lower end (non wireless) razer mice might also have button mapping. the software is probably similar to the one you are using now. i cant check since i'm not at my home pc right now (i have a deathadder)
Zowie EC2/EC1, Zowie AM, Razer Mamba, Razer Deathadder, Coolermaster Spawn, Coolermaster Xornet, Logitech g9x, Logitech 400. Those would be my top recommendations but your choice will vary depending on your grip style.
Take a look at the Razer Imperator..it's wired and very comfortable for long usage....But they do have quite a few mouse to choose from, so best bet if you can try several before buying. Razer, IMO, best mouse manufacturer.
Used Logitech, Microsoft before and I was not happy after awhile, either the button placement or design discomfort.