There's Sigma Chess (hiarcs' mac version), and shredder classic for mac, but these are substitutes for fritz, rather than chessbase. but they can handle databases too, I assume.
Mac Users? Chessbase-type programs

Your only real option is ExaChess:
Failing that, Scid has an OS X build as well:
http://cachivaches.chauca.net/scid/

SCID is very good, and you can configure it extensively. Then all you have to do is add UCI chess engines, such as Stockfish, which is available in a Mac build.
But you can also install Bootcamp, partitioned to run Windows on your Mac. Using Parallels or VM you can run both Mac OSX and Windows - but I recommend using Bootcamp (which comes free with OSX.) Then you just switch to Windows when you want to analyze, play or follow internet games/resources.
I user Fritz 12, and have installed a bunch of engines. Using Bootcamp, I use the full processors for the chess engine, instead of splitting the CPU between Mac OSX and Windows. Houdini, Firebird and Stockfish run with two processors on my Mac (Intel Core Duo.)

Scid is probably the best choice, but don't forget Jose, which is a very nice product too. Here http://jose-chess.sourceforge.net/index.html is the link. It has the nice feature of displaying the top candidate moves in the opening phase along with ECO codes, variation names, and win/loss/draw percentages for each move. You can also add UCI engines such as StockFish, which is currently the second strongest engine out there after Rbyka, and like Jose, it's free!
I'm considering making the switch from pc to Mac. I know chessbase doesn't make a chessbase 11 for the Mac, but do any of you Mac users have a comparable database program with a decent engine (rybka, fritz, whatever)?