Nice Productivity Timer
This timer has a great simple interface that makes it good for the "Pomodoro" method of task management. The local push notifications make it easy to continue the work and break periods even when the app is running in the background.
My requests would be including 12 hour time (am/pm), and adding an option to let the screen sleep (it can be a battery hog if not plugged in). Also, a "history" page to journal all of those productive work sessions would be very nice.
dfabs about
Concentrate! timer