I just started using my Droid to help me on my runs. I've found the Run.GPS is really good. It tracks me while I run, gives me average speeds, distance covered, calories burned. It also has spots for current speed and such, but since I'm running, I don't really look at that. It also has a ridiculous amount of sports to choose from. I also like that it syncs with a website so I can look at it and track my progress on a computer, any computer. You can also issue challenges and have "races". So far it's the best I've found.
I also like Soft Race. It does a lot of the same things, but I don't think it has a website, so it can't offer more of the social aspects as easily. And it doesn't have an area for you to put in body composition so it has a harder time with calorie counts. Still a great app if you're just looking for something to keep track of your runs.
I just downloaded SportyPal, seems to be really similar to Run.GPS. I think I'll try it out soon.
What are your favorite workout apps? Weightlifting, running, biking, hiking?