General
The RoadMap
Tips for ContributingChanges
ManagingBugs - how to work with the BTS
CompatibilityRules - how we keep our existing users from crying
OptionNaming - the conventions used to name command-line options
BasicCodingStyle - how to keep mpm from being grumpy with your code
WritingTests - how to add tests to the test suite
DeveloperRepos - where to find various public repositories of Mercurial work
NewFeatureDiscussions - A page of links to discussions about new features.
NewIdeas - Suggestions for features/extensions that would make someone happy
A list of Presentations and other related events.
MacOSXTools - Installing the tool chain on Mac OS X.
ProductName for the rules on using the name "Mercurial"
Mercurial Internals
DeveloperBasics - some basic code examples
MercurialApi - The public API for Mercurial
Design - Mercurial's data structures and how it uses them.
WhatGoesWhere - the project architecture
WireProtocol - the push/pull changegroup protocol
DebuggingFeatures - how to poke around in Mercurial's internals
Theming - how to create new styles for Mercurial's web interface
WritingExtensions - how to write Mercurial extensions
Old Stuff
First Mercurial UserSurvey.
The old ToDo list and MissingFeatures