CardioTrainer has my vote.
Out of all the apps I downloaded (God knows how many hours of sleep I've been losing playing around with that phone), I found CardioTrainer one of the most impressive programs on the Android. The current recession has left a bit of a dent in my wallet, and I wasn't really interested in paying for any applications (Buying the G1 was my self-made promise not to spend any more money this year on non-essentials), so CT being free made it a lot easier for me to dl and use.
Of course, just being free wouldn't be enough to make me like a program, but it at least allowed me to try it. Also, being a fit freak, I installed the program and took it for a spin. Its pretty cool in that it utilizes GPS technology and Google Maps in order to pinpoint exactly where I am running. It's also got voice commands, a race against yourself option (which I find probably the most useful part of the app), and best of all, its all in my phone so I don't have to drag a crapload of hardware around with me. When I'm done with my run, I can then check out my stats and route which is uploaded on their website, www.worksmartlabs.com/cardiotrainer/about.php