We have several mailing lists, including one for general discussion of MusicBrainz. This is an excellent way to get help or ask questions about this site. These are ideal for active discussions (and controversies!) with a wider audience.
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Likewise, if you've got an idea for something new or improved, please make sure it's not already on the list of outstanding feature requests before submitting a new RFE.
The MusicBrainz Wiki has a mix of discussion and documentation ranging from the introductory to the esoteric. It is primarily used for intermediate and advanced documentation of the MusicBrainz community, conventions, interfaces, and internals. While there's a fair amount of this on the main pages, over time most of is moving to the Wiki, where it can be maintained and extended by the community as a whole. The Wiki is especially well suited for documenting community issues and conventions like the style and editing guidelines, which change over time as a result of the community's interactions, and don't necessarily require development efforts.
The second use is for detailed and specific discussions. General ideas or opinions are best discussed on the MailingList, but once it gets to the point where there are detailed proposals (and counter-proposals) it makes sense to move discussion to the Wiki, where others can comment in-line and link to related topics, as well as summarize and re-organize the discussion. Additional discussion may still take place on the MailingList or IRC and if there is general acceptance of the suggestion, it may be a good idea to create a feature request on the SourceForge tracker.
The MusicBrainz team and MusicBrainz users are often to be found in the #musicbrainz channel on IRC chat at irc.freenode.net. Please feel free to browse the IRC log archives. The IRC channel is a very informal forum for Q&A or brainstorming, with immediate interactive discussions.
The Weblog has frequent updates from MusicBrainz developers on their progress in implementing new features and projects. While it has a comment feature for feedback, substantive discussion should be on the MailingLists or in the MusicBrainzWiki. Please feel free to link to our Blog from your own!
If you are editing the MusicBrainz database, be sure to use ModerationNotes on your changes, and on changes that you are voting on - your notes will be e-mailed to all moderators who vote on that change (or add a note themselves). This forum is very well suited for specific discussions about particular artists or albums.