Usability 
I love OpenTable. If you aren't familiar with the service, it's an online search for available reservations at local restaurants. You select your city, preferred dinning time, the size of your party and optionally the neighborhood and cuisine and OpenTable returns a list all the restaurants that have availability. It such a time saver compared with calling up restaurant after restaurant, being put on hold for five minutes and then being told "Sorry we have nothing available at that time".
The site is very popular here in the San Francisco Bay area and seems to be taking off elsewhere too. VentureBeat reports that OpenTable books reservations for 3 million diners per month and estimates the company's annual gross revenue at $14 million a year. The service is free to consumers, restaurants pay a fee per diner to OpenTable It must be cost effective for restaurateurs, 8500 have signed up.
OpenTable just launched a mobile site. This is such a great fit for mobile. Now you don't even need to be near a computer to make reservations quickly and easily. At least that's the promise. I just tried it out and successfully made a reservation on OpenTable mobile. The process worked but it has a few rough edges.