Oil rigs are pretty utilitarian structures, but once they've pumped all the oil they're going to pump, they become useless hunks of metal. If that seems more than a little wasteful to you -- not to mention an eyesore, considering how many rigs are within sight of shore -- you're not alone. But not to worry: industrious Ecogeeks are coming up with all sorts of ways to repurpose old rigs.
In this case, you don't need to do anything; the underwater support structures of oil rigs become artificial reefs on their own. There's one of these not far from my house in LA that's popular with divers called Oil Platform Grace, which has been inactive for years (but still serves as a pumping station). It was actually the site of a pretty nasty oil spill in its early years, but now all 320 feet of its underwater supports are teeming with fascinating life.