The guy did a lot more after that, but Alien is what he's most remembered for. I always thought of him as one of the founding fathers of modern SF in the movies — the post-Star Wars generation, eager to fuse spectacle and ideas in about equal measure.
Funny that this news should come the same week as the release of Avatar, which seems like the endpoint culmination of a lot of what he started — from O'Bannon to Cameron by way of Alien(s).
My favorite O'Bannon story — there's a bunch of them — comes from an interview he gave with OMNI right before Alien was released. He'd suggested that Ridley Scott see The Texas Chainsaw Massacre as a possible point of inspiration for the film:
... I was sweating bullets the whole time. I kept thinking, Oh, he's British and he's got very refined sensibilities. He'll hate it and think I'm a jerk. And then he came out raving. He said, "Alien's got to be like that but better!"
... I know what people are gonna do. They're going to go into the film and tap their toe and say "Okay, show me the monster, show me the monster." And when we show you the monster, you will jump up and run out of the theater.
Good night, Dan. You were a game-changer.