Everything you need to get started is in The Definitive Guide. I installed Mercurial via 'port' on OS X Leopard and had an issue when trying to set up the web system for it. If you receive the following error:

No module named mercurial hgwebdir

Then edit your hgwebdir.cgi file and near the top add this:

import sys
sys.path.insert(0, "/opt/local/lib/python2.5/site-packages")

My web server configuration was created in /etc/apache2/other/mercurial.conf

ScriptAliasMatch ^/mercurial(.*) /Library/WebServer/Documents/mercurial/hgwebdir.cgi$1
<Directory /Library/WebServer/Documents/mercurial>
  Options ExecCGI FollowSymLinks
  AllowOverride None
</Directory>

Mostly following the guide from http://www.selenic.com/mercurial/wiki/HgWebDirStepByStep

GUI Clients

Murky on OS X

IDE Integration

• Eclipse