I was surprised that FanimeCon wasn't already posted. In any case, it's basically the San Francisco Bay Area's anime convention (you can find their website here). The Con is usually in late May and is pretty fun (San Jose -- where it takes place -- is full of Asian people, so the Con has a lot of really cool Japanese cosplayers amidst the pastie white guys).
However, once you've been to one Fanime, you've been to every Fanime. -_-" It can be pre-tty repetitive... But it's a fun one-time thing, in any case.