Rule Development
At the moment we do this through BugZilla; a rule that looks good is taken by the dev team and checked into SVN in the developer's sandbox (see RuleSandboxes), whereby a distributed group of contributors will test it against their corpus in the NightlyMassCheck and PreflightBuildBot mass-check runs. The resulting collated accuracy data can be viewed at the RuleQaApp.
MassesOverview describes a number of tools that are useful in evaluting the quality of prospective rules.
CustomRulesets illustrates rules available from third parties and has links to their mass-check results.