C25K I started with this app, which is basically a glorified stopwatch. It plays a beep over whatever music you choose to play on the music app of your choice, indicating when it's time to run or walk. Simple, and free.

Ease into 5K This is the Android port of the super-popular iPhone app for this program. It definitely has a spiffier interface, and it lets you upload and share your workouts on Twitter and Facebook.

C25K Couch to 5K by RunDouble Tracks distance and pace, like the above app, and gives you a 2-week trial period, which the C25K app above this one does NOT have; you pay up front for that. Save a few bucks if you're not sure if the program is for you, and try this app out.

adidas miCoach This isn't a Couch to 5K app, but it's a good general fitness app for customized training plans - especially if you don't think Couch to 5K isn't for you. You can set goals and target training for specific sports (you'd train differently for basketball than for distance running, of course), and it has a neat interface with Adidas online. Plus you can set your trainer to sound like Reggie Bush, and that's hot.