Linux: http://eyedeal.team88.org/node/86 (uses Workrave, http://www.workrave.org/)

Linux + Adobe Air: http://code.google.com/p/pomodairo/

I didn't find anything nice for linux when I was looking for one a few days ago.

I ended up buying a docking station / stand for my ipod touch, and using the Pomodoro Timer from Navel Labs. I like having it on a seperate tiny screen so that I don't have to go hunting for the application whenever I want to see how much time is left, etc..

I wrote a tiny little thing (61 sloc) with Shoes that is cross-platform. http://github.com/jroes/pomodoro

I like Timer Applet for Gnome.

https://launchpad.net/timer-applet