Yes, there are at least a trillion cyclocomputers from which to choose. But over the years, I've found that CatEye's ones are just about bulletproof. In fact, I don't believe I've ever had one failexcept for the one that got ripped off the handlebars while I was doing an endo not far from here in the Olympic National Forest. But that really wasn't the computer's fault...
Anyway, the MC100W ($50, www.cateye.com, though currently $35 at Nashbar; www.nashbar.com) is a wireless unit that should be just right for you. It has all the usual computer wizardry, plus an LED backlight and the ability to load it with two sizes of tire, a handy feature if you ride different bikes. Or, as I do on my mountain bike, you put on fatter tires during the winter for more "float" in mud (and, oh, has it been muddy!). You also can mount it to the stem or the handlebars, which is a nice option. CatEye's CC-CD300DW ($150) ups the ante by adding a slew of training and heart-monitor functions. In fact, its screen offers so much information I'd probably plow into a tree while trying to figure out my coefficient of drag squared by my average heart rate over the past 17 laps.
An interesting newcomer to the domestic bike-computer scene is VDO, known to sporting motorists as a fine maker of automotive gauges. It's now offering bike stuff, including the wireless VDO C2 DS ($70; distributed in U.S. by Ibex Sports, www.ibexsports.com), a nice-looking unit with all the features of the MC100W in a slightly more stylish package.
The rationale for using a wireless computer on a mountain bike is that a wire runs the risk of snagging on brush and branches. Which I buy in theory, but in practice I've never seen it happen. So I ride with a tried-and-true CatEye Enduro ($26). I've even seen them for around $16, a fabulous bargain. So that's yet another alternative.
For more mountain-biking gizmos and gadgets, check out Outside Online's encyclopedic MTB gear page.