OnlineClock.net is one of those simple, classic websites that just about everyone knows. The greatest thing about this website is its simplicity: it's not filled with a lot of extra clutter.
You just go to the Online Clock website and select your alarm time, and you are finished! The site functions just like an alarm clock... hence its name: The Online Alarm Clock.
I don't really use it to wake myself up in the mornings, but I use it almost every day to remind myself to do stuff while I'm online. It's a great free service for online reminders.