Crate engines...
 

Those crate engines are some of the best deals going. Very big bang for the buck.

However, IMHO, if you got one, I would have a shop go through it. Since marine engines run a little cooler than their automotive counterparts, they should have a little more clearance on the piston-to-cylinder wall and valve guides. I've heard horror stories about forged pistions skuffing cylinder walls, and valves getting hung up...probably due to the engine running cool. Sometimes they run fine, but it's cheap insurance.

Either way, you can't go wrong with a balanced and blueprinted crate engine.