Xenharmonic Wiki is an open resource repository for information about musical tuning. This includes information about systems of tuning, the relationships between those systems, and the theory and math behind them. It specifically targets microtonal music, xenharmonic music, alternative tuning practices, et cetera.
The wiki, when originally launched in its original incarnation, was admittedly vastly incomplete, but has been added to by volunteers only when doing so is useful to them. New content is most welcome (as long as it's relevant).
Xenharmonic Wiki was originally created on Wikispaces sometime during 2005. During 2017, Wikispaces announced that it would cease all wiki operations, and all its hosted wikis would be closed. It gave a lengthy notice period to this effect, and also created XML backups of all individual wikis, which could then be downloaded and resurrected elsewhere on another host with a compatible wiki engine. The web archiving service archive.today at archive.is indicates 56 captured URLs of the former Wikispaces wiki; whereas the InternetArchive WaybackMachine at Archive.org lists 25 captured URLs at Xenharmonic.Wikispaces.com/space/, and just 3 captured URLs at Xenharmonic.Wikispaces.com/wiki/, though an impressive 2420 captured URLs at Xenharmonic.Wikispaces.com/. Xenharmonic Wiki subsequently chose independent hosting using MediaWiki, and its own URL at En.Xen.wiki.