I often share the tools I use to get my one hour of writing a day in the can. Here's one I've found I can't live without;
I've experimented with a few timers in the past (and time trackers like Toggl), and this is the best one I've found so far.
PROS: You set up an Interval (I like to do two 25 minute intervals with a 5 minute break), hit Go, and the rest is pretty self-explanatory. An alarm goes off when the interval is over, then gives you 5 minutes to surf the web or do one armed push-ups, then you're back in the saddle again. Boom. You nailed it.
CONS: I wish you could set up different intervals for different projects. Plus, I wish you could set your own custom alarm. That way, I can blast the Game of Thrones soundtrack every 25 minutes to pump me up (and thoroughly annoy those around me).
Got any tools you're using right now you love? Lay them on me.