Well, we can now make a new item mob grinder.

The Iron Golem grinder. It will take up less space and is easier to build. (IF, you have the Iron..)
I plan to make one, after Bukkit updates..

I've already read about this, and I think it's going to be a bit over-the-top. Probably send iron prices through the floor... but I still plan on making a farm of my own too.

Don't really need any iron to make it. Go out and turn a bunch of villagers into eggs, I think you need 15, so maybe buy them form a shop. Place them in a house wit ha door, but make sure they cannot open it by putting glass on the inside of the door. The golems appear on the first block outside the door, that would be the entrance to the grinder/farm for them. Fill it with water and let them drown, cause they are tough.

That could work but I meant for stuff like creepers and zombies.

Iron golem grinders sound awesome. You would have so much iron but you would still need to pay a lot for the 15 villagers.

Oh, using them to do the killing? Sorry, misunderstood.

They don't attack creepers.

Zombies and skellys...