There are many threads on GPS units as alternatives to cyclometers.

Some brief things to bear in mind, though: GPS units lose contact with satellites occasionally, whereas cylcometers keep constant measurements. GPS units tend not to tally the wiggle of your wheels as you ride-- the unit straightens out your path in calculating your distance so there is about a 1% error compared to a well-calibrated cyclometer. GPS units eat batteries. Cyclometers do not.