Rush, King Crimson, Pink Floyd, some of the early progressive rock bands. I'd mention Yes, but I can't stand their singer.
Edit: Someone mentioned Porcupine Tree, which is one I forgot. They are great, a good mix of really hard rock with some progressive writing. Funnily enough they opened for Rush several years ago, though no one has opened for Rush since the R30 tour. Porcupine Tree are really worth checking out, I highly recommend The Incident, some of the tracks remind me of Moving Pictures with the synth work on the album.