Interval Clock guides you through interval training with a countdown timer that alerts you when you have to go faster or slower. This is ideal for use on a stationary bike, treadmill, or other piece of exercise equipment that allows you to look at your phone (although you can use the app for just its audio alerts without machinery if you prefer).
The app originally worked for a 20 minute workout with short intervals, as this was found to be an ideal way to train (See this Lifehacker post for details: http://lifehacker.com/5989669). Now the app supports interval lengths and timers of your choosing. It will also remember the last interval you chose so you don't have to change it frequently.
If you have any issues or want to report bugs, please let me know and I'll do my best to fix them.