Timer App
I realize I'm a bit late to the party, but I still wanted to chime in. There are various apps that handle multiple countdown timers, which are helpful if you are running multiple events concurrently, such as maby being in charge of two or three GPTs on a GP Friday. Some have been mentioned, another on is called “Chronomet”; it will handle a variable number of concurrent timers (I guess there might be a limit, but without checking I'm not aware of it. If it exists, it's certainly higher than 3, though).
For main events of large tournaments, such as GPs, though, I'll recommend “Ultrachron”. While it offers only one countdown timer, it is the only countdown timer app that I'm aware of that will keep counting after hitting 0. I.e. it will allow you to easily track the extra time given to the table you are sent to cover “about 1 or 2” minutes after the round has actually ended.