Tomatoes

Tomatoes allows anyone to log in with a GitHub account and start tracking their pomodoros.

pro Use anywhere
Good if you have more than one machine or work in different locations.
pro Graphs
Shows what time of the day you do the most work. Great for planning the best time to take a break.
pro Leaderboards
See how you compare with different uses, based on the day, week, month and all time.

con Requires a web browser to be open
Can be a problem on low-memory systems.
con Requires a GitHub account to really get the most out of it.