Dungeons of Dredmor- gorgeous graphics, active developer support, $5 on Steam, with an expansion pack for $3. Works just dandy on the Mac, he even lets you command-click instead of control click, and I'm sure you know how much nicer that is.
SLASHEM(SuperLotsaAddedStuffHack-ExtendedMagic) and it's graphical version (Vulture's Claw?) are of course worth mentioning, as is Binding of Isaac, which is made by the folks at Newgrounds and is real time, IVAN would be great if the game wasn't quite so obstinate at not letting you win (literally the only way to win is to cheat),Unreal World is a survival-y one, and there's Cataclysm, which is great but getting it to run on the Mac is a massive pain in the ass. It's a zombie survival thing.
Roguelikes are the best games, in my opinion, especially the ones that break away from the typical ADOM/Rogue/Whatever formula and experiment with genres and graphics.