Linux
A good GUI for Mac

There are many. What kind of use for?
Scid vs Mac; Chess King 2015; HIARCS Chess Explorer; ExaChess; Shredder Chess.

1. All of them are very good in game analysis. Best here being probably HIARCS Chess Explorer.
2. Playing the engine, the best is probably Chess King, but HIARCS and Shredder are very good too.
3. Engine Tournaments. Chess King don't do engine tournaments. All of the others can. Scid vs PC can even draw the tournament diagram.

Join us in this thread: http://www.chess.com/forum/view/general/scid-vs-pc
Scid vs PC and Scid vs Mac are the same GUI.

There are many. What kind of use for?
Scid vs Mac; Chess King 2015; HIARCS Chess Explorer; ExaChess; Shredder Chess.
XBoard is available as OS X App, packaged with several engines. It is available from WinBoard forum.
http://www.open-aurec.com/wbforum/viewtopic.php?f=19&t=53274
It is an all-round GUI, good at analysis (it supports multi-PV and move exclusion), opening-book building, engine tournaments (supporting concurrency) and has a reasonable database functionality (position search by various criteria). It comes with a beatable engine that you can play against.
Does anybody know of any names?