SimpleIntervals Online lets you time intervals for workouts, right in your web browser. Start it and you get 20/10 intervals with optional sounds, perfect for doing Tabata interval training.
If you like SimpleIntervals Online, give SimpleIntervals Mobile a try on your iPhone or iPad. It's a mobile app that lets you select interval times, use sounds, and has the same visual ticker as the online version. Click the link below to view the app in iTunes.