[http://www.perforce.com/perforce/products/merge.html P4Merge description], [http://www.perforce.com/perforce/downloads/index.html Download page]

P4Merge is a 3-way merge tool which comes with the Perforce VCS. It is quite powerful so on OS X (where KDiff3 requires running X11) is likely the best available tool (TODO: compare with [:DiffMerge]). To get it you should download "The Perforce Visual Client". In OS X case you get p4merge and p4v applications, so you need only first. Install in in Applications and then add to your ~/.hgrc: 

[merge-tools]
p4.priority = 60
p4.premerge = True
p4.executable = /Applications/p4merge.app/Contents/MacOS/p4merge
p4.gui = True
p4.args = $base $local $other $output
p4.binary = False

Adjust priority based on your other merge tools (or just leave it if you have no other merge tools) and path if you're not on OS X.