Can't help you with the free stuff, but I can point you to some cheap stuff:
Darkstone (almost as old as Diablo, but I really liked the ability to play 2 characters at once).
Sacred (prettier graphics, weaker game play, much bigger world)
Divine Divinity (I couldn't get into it and never finished, but some people really liked it)
Dungeon Siege (not really a Diablo-clone, but if you haven't played it, it's very good. Get the expansion, as it includes the original)