There's a UB of SheepShaver, though I have no idea how well it works. The PPC version works fairly well for the most part, though the graphics handling needs serious optimizing, so it's not so good for games (though it'll run System Shock just fine, which Classic won't do). You do have to buy a copy of OS 8.6 or 9.0 though...it doesn't work with anything newer last I heard.
--Eric