Right now, BP is trying this "top kill" procedure to stop the flow of oil into the Gulf of Mexico. Here's an animation showing how it's suposed to work.
And here's another one:
Neither of these is very good, I know, but the basic idea is this: You inject a heavy fluid into the blow out preventer (which sits on top of the well site) and it flows into the well, slowing down the oil. Once it's slowed down, you follow that with an injection of cement and you've stopped the flow for good. Here's a more thorough and technical description.
There's also a live video of the top kill procedure as it happens but it's quite hard to figure out what's going on.
And for updates on whether it's working, check the (real) BP America Twitter account.
UPDATE: This short slideshow from the BBC is the most efficient explanation of the top kill procedure I've seen.