Are you interested in just the basics or do you want bells and whistles? Do you have a MTB and are you worried about it surviving harsh conditions (including offroad crashes)? Do you care about interfacing it with your bike's shifters? Do you care about having an altimeter? HRM? PC interface? Compass? Backlight? Want to customise the display? There are plenty to choose from. I've seen a really nice comparison chart for various computers in Performance's catalogs. Of course that's only for computers they carry but they do have a pretty good selection. You might wanna start there.

Me? I have the Shimano Flightdeck for both my RB and MTB. It interfaces with my shifters and shows me gear information. It also shows virtual cadence. The Ergobrain for the Campy groups do the same but show actual cadence and have backlights. They also "learn" the gears instead of having to be programmed by hand.

My wife has a Cateye Velo (basic computer) on her bike. I also like the Cateye Mity and Enduro (ruggedised version of the Mity). I used to have a Cateye ATC (no longer made). A friend of mine has the Cateye OS which allows you to reconfigure the display and stores summary charts of your riding.

If you're looking to combine HRM, cycloputer and total training program, you might wanna look at the high end Polar.

Most of these computers can be purchased as a wireless unit or can be upgraded with a wireless kit.