While it doesn't have a very catchy name, Minecraft snapshot 12w36a features quite a few interesting additions. Take this fellow, for example. The Wither skeleton has a sword, instead of the usual bow, and he's so excited to show you his new toy. Should his weapon make contact, your health bar will turn black and your life will drain away like so much bath water down a plughole. Lovely stuff.
The snapshot also introduces the Nether Star, which is dropped by defeated Wither bosses and which is a vital ingredient for the new beacon blocks. Be careful, though, because those Withers now regenerate their health, as if they weren't already tough enough. You'll know when they're around, too, because the sky now darkens. Eek.
Other major changes include new breeding methods. To breed chickens, you'll now have to feed them seeds, while pigs will scoff carrots. Also, there's a small chance that mobs will drop heads when they die. What can you do with these? Well, wear them, of course.
If you want to give the snapshot a try, check out our guide here, but do backup all your Minecraft stuff first, just in case.
Credit due to redstonehelper on reddit, whose screenshot we've used. Check out redstone's thread for a more in-depth analysis of this snapshot and for even more pictures of unpleasant monsters.