mercurial
The general mailing list. Busy, but has a high signal-to-noise ratio:
Mailing list archives, searchable on Gmane, MarkMail, marc.info and nabble.com
mercurial-devel
The developer mailing list. Created in October 2006:
Mailing list archives, searchable on Gmane, MarkMail and marc.info
mercurial-packaging
This is a low-bandwidth list is for people who build distribution packages of Mercurial to coordinate releases and also discuss packaging-specific issues.
Created in Feb 2010, see also Matt's announcement: http://selenic.com/pipermail/mercurial-devel/2010-February/018655.html
Subscribe here: http://selenic.com/mailman/listinfo/mercurial-packaging