Ambisonia was a site set up by Etienne Deleflie containing much practical and theoretical information about Ambisonics. It also hosted torrents of ambisonic recordings contributed by a number of people (including myself), as B-format files (.amb) and as G-format files encoded to DTS. Ambisonia has now closed down, but Etienne is developing a replacement called Sound of Space (qv).
I have made all the recordings that I put on Etienne's sites available here, and also John Leonard's, with his kind permission. Both .amb and .dts files are provided.
The BLaH trilogy is a group of papers about Ambisonic decoding - based on listening trials - produced by Eric Benjamin, Richard Lee and Aaron Heller.
The Gerzon Archive - Keith Howard has scanned and made available a considerable number of Michael Gerzon's papers, including some of the earlier ones which were previously hard to find.
Stephen Thornton was around with Michael Gerzon from the earliest days of his work. This website gives an account of those days, copiously illustrated with photographs and notes from Steve's diary. (Also a few photos by me and of me.)
Sound of Space is a new website that Etienne Delaflie is developing to replace the now defunct Ambisonia website; it is currently still in beta, but already has a modest amount of material on it.
The B-format files are in a new Universal Ambisonic format, for which there are at present no players, as far as I know. G-format files are available in .m4a and .ac3 formats, which are playable on many systems.
The sursound mailing list is an active list discussing all aspects of surround sound. Although at times it seems to be almost solely concerned with ambisonics, this is not its intention, and other surround techniques are discussed from time to time.
There is an article on Ambisonics in Wikipedia. It has been extensively updated recently, mainly by by Richard Elen and Martin Leese, and contains links to related articles.
I recently discovered a short-lived record label, all of whose fifteen recordings (mainly of organs) were made ambisonically using a single SoundField ST-250 microphone and released as UHJ stereo. I have documented the recordings that I have been able to acquire or find out about on a separate page here, as the information is not yet available in the UHJ discography.