In a pinch, you can Google something like “five minute timer” and have a timer right in your web browser you can use.
But maybe you’re not in a pinch. Maybe you’re looking to create some ambiance. If that’s the case, I recommend OnlineClock.Net. Sure, the front page is a very pared-down digital clock, but there are more options. You can customize it with sounds and backgrounds, like an aquarium or fireplace. You can change it to a stopwatch or a timer. There are pre-set timers for major cultural events, like the upcoming New Year or Superbowl. You can even play a radio station. There’s even a games section if you’re just looking to kill some time.
I like to put this website up on the Smartboard with the timer on to remind students when it’s time to log out and leave the lab — my classroom is one of the few in the building without a wall clock, so it’s been very useful, especially for my after-school group.