Asimov and Bradbury are the most famous building blocks of the genre, of course; if you're writing anything involving a robot, Asimov's the place to start as he basically codified robot behavior in science fiction. One name I don't hear tossed around enough is Ursula LeGuin, who was a master at world-building and species-building (if that last part makes sense) in her novels and especially short stories.

Space travel is something I think will be restricted to fantasy for a very, very long time. As it stands, it's more likely an alien species visits us than we visit them, and the odds of this species being from a terrestrial-type planet are pretty low. That said, Star Trek is tons of fun to watch, so as a writer the idea of visiting a very very foreign land still intrigues me. I admit it's not my strength, though.