apple wood and cherry wood are pretty common. Coffee wood has also been used. most other woods that are used aren't as hard, so working them is going to be a little different.
I've heard that Zebrawood is used as well. I actually ordered a 6x6x3 block and am going to try it out. On back order of course and won't be delivered til some time after 11/30. Enough for 6 pipes.