Bundled extensions
1. Core extensions
UsingHgk - how to use the GUI repository and history browser
MqExtension - Mercurial Patch Queues - manage changes as series of patches
UsingBisect - how to use the bisect extension to find bugs
UsingExtdiff - use external programs to compare changes
AclExtension - Manage commit access to parts of a repo using control lists
BugzillaExtension - Update bugzilla entries when a bug id is referenced in a changeset
FetchExtension - Conveniently pull and merge or update to the tip revision
GpgExtension - Sign and check changesets using GPG
NotifyExtension - Email notifications to subscribed addresses after commits to a repo
PatchbombExtension - Send a collection of changesets as a series of patch emails
2. Additional extensions
ChurnExtension - Show statistics for mercurial operations
PurgeExtension - Purge all files and dirs in the repository that are not being tracked by Mercurial
Non bundled extensions
ForestExtension - manage a bunch of mercurial repos as a meta repository, with snapshot support
TransplantExtension - cherry-picks patches and rebases branches
KeywordExpansionExtension - use CVS like keyword expansion in tracked files
See CategoryExtension for a more complete list and ExtensionHowto for a list of bundled extensions.