NPR has published a list of the top 100 science fiction and fantasy books. I've read 65 (many of them for this blog), though I'm counting a series even if I've only read one book (hello, Thomas Covenant).
I can't say I'm thrilled with everything on the list. For one, it seems heavy on Tolkien rip-off fantasy series. But, these things are always fun. It's nice to know there's plenty of material out there for me to work through once I finish with the Hugos, Nebulas, Locuses, etc.