My gaming group likes to cycle through different games. Aside from D&D, others that we've played and enjoyed include Space: 1889, The Burning Wheel, All Flesh Must Be Eaten, and Call of Cthulhu. We've played others that we haven't enjoyed so much (Shadowrun, I'm looking at you), but I'd recommend any of them very highly. I find it's good to introduce a variety of rules systems and settings to the group; it keeps things fresh, and allows you to import ideas from other systems into D&D.
Also, I'm one of those guys who will buy an interesting book or supplement from another system even if it doesn't seem likely that we'll play it, just because I love reading them. I have AEG's The World's Largest Dungeon, West End Games' D6 Adventure, Frank Chadwick's Cadillacs and Dinosaurs . . . so, yeah, my bookshelves groan beneath the weight of a variety of systems, too.