You didn't say what program you were using. MAX has a nice noise generator which I use to create rocks, that does pretty much what you want. Plus there are a couple of free plugins that can further distort the mesh. If you don't have a modeling program that allow this, you might try downloading Wings, as mentioned, and modeling in that. It's a decent program, and if, as it sounds, you're using Bryce, it could expand your possibilities immensely.

Talent is God's gift to you. Using it is your gift to God.