This stuff will most likely not be terribly useful to those not interested in SpamAssassin development:
Introduction
Generally, we coordinate the overall flow on the SpamAssassin-Users and SpamAssassin-Dev MailingLists. Then, for specific new functionality or rules, a [http://bugzilla.spamassassin.org/ Bugzilla bug] should be created to track progress and provide a forum for discussion; any patches should be stored (as an attachment) on that Bugzilla bug.
The code itself is maintained in a [http://subversion.tigris.org/ Subversion] repository which can be browsed via [http://svn.apache.org/repos/asf/spamassassin/ HTTP] and WebDAV (replace the http scheme with webdav to browser the repository – [MoinMoin] doesn't like webdav-links). There are also [http://svn.apache.org/snapshots/spamassassin/ snapshots] of the current development tree available.
General stuff
- RuleDevelopment: some notes on rule development.
- UsingBugzilla: What to watch out for when using SpamAssassin's bug tracker.
- CodingStyle: To be written.
- NextRelease: Might contain a schedule for the next release.
- DownloadFromSvn: How to download the latest version of SpamAssassin from Subversion.
- VotingProcedure: We follow the ASF way of doing things like this.
- DevelopmentMode: The difference between C-T-R and R-T-C, etc.
Stuff relating to the Incubation process
- IncubatorToDo: To-do list for moving out of the Apache Incubator
- ClaVettingProcess: How we tracked down code owners in order to relicense, and current status.
Stuff about scoring
- TenFoldCrossValidation: How to test the accuracy of the scoring system
- NightlyMassCheck: How to run nightly mass-check runs to help with rule scoring
- RescoreMassCheck: How to generate a new score-set (the big mass-check run)
More Stuff
- MailManglingInTheField: Information about known ways a mail message can be changed in transit.
[http://spamassassin.apache.org/publiccorpus/ The SpamAsassin public corpus]: an old collection of hand-filtered ham and spam for testing GA work, developing new filter techniques, and benchmarking filters.
[http://spamassassin.apache.org/gtube/ GTUBE]: the Generic Test For Unsolicited Bulk Email.
- Package Building: RPM's, etc.
I'm sure more stuff should be added here. Please help out 
(TheoVanDinter)