I certainly know what you're talking about! Over the weekend, I cleaned a giant litterbox..........my horses' stall. People haven't been cleaning it like they should and it was horrible.
I just thought of something. Go here: http://www.sweetpdz.com/
This is the stuff that you should get to put in the bottom of the litter box. You can get it at a feed store. I would check with your vet to make sure it's safe for the cats first, though.