Just bought a Logitech wireless trackball. It's going to take some time to get used to, but I like it so far. I have a feeling I'm going to like it a lot more on my laptop and in instances where I have to jump between monitors a lot. I love my Cataclysm mouse, but it is kind of on the huge side.
Edit: I've been using it extensively for the last couple of days (not for games, obviously; I use my million button mouse for that) and it's been working out great. I think I would have preferred one with a finger-controlled ball, but those tended to be more expensive and didn't fit the shape of a hand particularly well, appearing to be designed more for mobility than comfort. The one that was a reasonable price -- another Logitech one, in this case -- had an odd button layout. However, the thing that killed the deal was the size of the wireless receiver. It was one of the large ones with a cord. The one that I wound up buying (the M570) has one of those nubby-looking receivers that Logitech has been using lately. The nice part about that is that I can stow it away in the battery compartment if I should want to bring it with my laptop. There's even a little slot for it there.
It's funny, but I think that this is the first trackball I've used on a home system in around twenty years. I used to have an AT keyboard with a 3 button serial trackball built-in many years ago. I doubt I still have that.
Edit 2: The weirdest part, so far, has been trying to train myself not to move my thumb when I left-click (or to take my thumb off the ball before I click). Thankfully, that's proven easier to do than expected.
And finally, unlike the last two Logitech pointing devices I've used (a G5 mouse and an MX518 -- the former having been shitcanned because it's a cheap, horribly designed piece of shit and the latter being used on my Mac), this one has a scroll wheel that doesn't suck. Seriously, this is the first Logitech mouse that I've had that has had a clicky, responsive wheel since my MX510. The G5 and MX518 would both double- and triple-scroll occasionally, making them useless for just about anything related to gaming, and the G5's scroll wheel eventually gave out altogether. The M570's scroll wheel is a bit firmer than the one on my Cataclysm mouse but is equally responsive. Thank you, Logitech, for both realizing that what you had sucked hard and working to correct it.