https://www.mercurial-scm.org/repo/hg-stable — the stable repository, solely managed by mpm, contains official releases with safe and important fixes applied, always a subset of the main repository
Bugfixes are committed to the stable branch and pushed to the main repository. This branch is automatically pushed to hg-stable to provide a separate repository. New features go to the default branch, which gets merged into stable at the start of each code freeze.
Other core repositories
https://www.mercurial-scm.org/repo/hg-committed/ — where core contributors push draft changes for the main repository
https://www.mercurial-scm.org/repo/hg-all/ — combined mirror of main & hg-committed
The flow between the main and committed repositories is shown in this graph. The blue repositories are controlled by Matt, the dark green repositories by core contributors and the light green repository by MartinGeisler.
See the TortoiseHg Wiki page for that project's development repositories.
Repositories that have been decommissioned or had no recent activity:
http://hg.netv6.net/clowncopter/ – Where Facebook main contributors used to queue changes for the main repository,
http://hg.intevation.org/mercurial/crew-stable – changesets of the branch named stable are pushed here automatically,
http://hg.intevation.org/mercurial/crew+main – automatic mix of crew and main (may contain multiple heads),
http://hg.intevation.org/mercurial/tah – This repository now contains the latest version tested and deployed by Thomas Arendsen Hein. His outgoing changes go either to the devel list or directly to crew, so it is listed as inactive (for development purposes).