Grado SR-60s. They sound great.

Before that I was using a pair of Sennheiser HD-280 Pros, which cost like three times the price, but I don't think sounded much better. It's not a fair comparison though, because they were so uncomfortable and fever-inducing, I couldn't really evaluate the sound quality. (Also I wasn't using an amp, but I've never met anyone who uses a headphone amp and isn't a total geek, so I think that's something I want to steer clear of.)