Simple Machines is actually the next evolution of YaBBse, available at http://www.yabbse.org
. If you like the look and feel of SMF, you might give YaBBse a try.
Personally I've tried a few, and have stuck with YaBBse. There are other excellent forum packages available, and some do have features not planned in YaBB until the SMF release (phpBB comes to mind).
Edit: fixed link