These are good for mud, and have a little volume so you can run the pressure lower with a DH tube = good for muddy to dry/loose
http://www.maxxis.com/bike/productDetail.asp?BrandID=217it looks a bit mislabeled - top 3 listed are with DH casing, bottom 3 with standard casing. Can certainly get better prices elsewhere.
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These are killer for nasty wet mud and running water/active rain - like plattekill - run them at 30lbs so they can cut through the slop to the grips + they are thin (2.2) so they dont float/slide on mud.
http://www.maxxis.com/bike/productDetail.asp?BrandID=46---
Also check out Intense Spikes - 2.25 similar to the wet scream, or there is the Arrow Mud-X, but I dont like that it has center knobs for mud - the swamp thing is similar but centers are slightly offset.
If you want to use them just for sloppy mud (esp PKill in the rain) try the Wet Scream or the Spike - other 2 are more general use but good.