Oh yeah, my second favorite mod...
Have to be the TKC-80 tires. They are real confidence builders when you get offroad. And, the best part is they work really well on pavement too.
They are a little pricey and from what I read they don't last as long as other tires. But, I've got about 2000 miles on them so far and I think they should go for another 1500-2000 miles. I'll buy them again when the time comes for new rubber.
Others like the D-606's, but I haven't tried them. I figure I'm happy with what I've got, why experiment further?