iPhone: It seems like no one wears a watch these days; everyone just uses their phone's clock. For those times you can't pull out your phone, there's Buzz Clock, which vibrates on schedule to keep you informed of the time.
We've talked a bit about mobile phone etiquette here at Lifehacker, and it's clear that some people find it just plain rude to pull out your mobile phone at certain times — even if you're just checking the clock, it makes it look like your distracted, checking for text messages or missed calls. To keep track of the time silently when you're at dinner, in church or wherever, Buzz Clock will vibrate on an interval so you don't even have to pull it out of your pocket.
The app is pretty simple; you just set an interval on which it can vibrate (5, 15 or 30 minutes) and forget about it. A half hour seems like just about the perfect interval; it's accurate, but won't be buzzing so often that it drives you crazy. Hit the link to check it out.
Buzz Clock is a free download for the iPhone.