I'm searching for compelling Git and Mercurial clients on Mac OS X. The most clients I've found so far were less compelling as I expected. Some of the clients are programmed even in Ruby or Tcl/Tk, which IMO aren't good OS X citizens in regard of integration in the OS.
I have clients similar to Versions.app or Cornetstone in mind, which are Subversion-only clients. Perhaps somebody got an insider tip for me.
For Mercurial, you should take a look at MacHg. It's free and open source. It uses the native GUI toolkit for Mac and comes with its own bundled version of Mercurial. It is very polished:
(source: jasonfharris.com)
There are many more screenshots available.
I've also heard many good things about SourceTree, which is both a Git and a Mercurial client. Atlassian (owners of Bitbucket) bought it recently and are now offering it as a free download.