Is there a Mac OSX implementation (or equivalent) of WinBoard / XBoard?
You can technically still use XBoard, but you will need to install X11 from MacPorts, since IIRC Apple no longer ships it.
Julien Marcel has a collection of well-compiled OS X chess engines (which do not require emulation).
As for not having a ChessBase client, HIARCS Chess Explorer does fine.
(FWIW, there is a port of XBoard to Cocoa - currently named OSXBoard - but it seems to have stalled.)
Jin seems like a good option.
Works well on OSX 10.8, and compatible with other platforms as well.
Jin is the best, also on older macs there was another client "Fixation". I would recommend using a virtualization application such as VMWare Fusion or VirtualBox with a Windows guest to run your preferred Windows chess software - as macs also suffer from not having a satisfactory ChessBase client to review games. Performance is not an issue as chess apps do not need be too powerful for these things.
There is now a native quartz xboard.app through GTK2. http://www.open-aurec.com/wbforum/viewtopic.php?f=19&t=52964