Round Timer

http://www.onlineboxingtimer.com/

I like that you can customize the settings on this one. There doesn't seem to be any sound though which is bullshit but a lot of times when I'm training at home it's while everyone else is asleep anyway.

I found this one on www.PrecisionStriking.ca
http://dl.dropbox.com/u/4633554/Boxing/Boxing%20Beats/RingTimer.mp3

Something I like to do is to play the above mp3 in Windows Media while running a metronome on Youtube at the same time. Just look up "metronome" and there are lots to choose from. The faster the better.