I’ve been trying to keep a useful account of the state of what’s where at the Inform 7 Documentation and Resources page.
The Public Library extensions are still the canonical source of extensions as used by the IDE, but it has not been updated in some time: there are more recent versions of many of the extensions there, and I spot at least one that doesn’t work in 6M62 (Remembering by Aaron Reed). (And Mantis and uservoice are decommissioned. And a bunch of stuff that used to be at inform7.com isn’t there any more.)
The comprehensive community-maintained collection of extensions is at The Friends of I7 Extensions github repo, with old ones available at the Friends of I7 Extensions Archive. This is an easier-to-browse view of the extensions repo but it isn’t maintained automatically and I haven’t updated it in a few months, so some things are missing.
There has never been any requirement or expectation that everything in the Friends of I7 Extension archive works for any given version and about a third of them don’t compile. The non-functional extensions usually aren’t marked. I have a prototype extension listing that at least separates things that do and don’t compile on 6M62, but I’m in the middle of a large revision and don’t know when I might be ready to propose putting it on the repo in place of the existing easy-to-browse view.
A bunch of major extensions are no longer maintained by their original authors, like Emily Short’s and Jon Ingold’s. A bunch of people have commit access to the git repo and so are capable of changing anything, which sounds like a recipe for disaster, but in practice it seems to work well and if anyone has been upset by a change someone else committed, I’ve never heard of it.
[Edited: per David Kinder’s correction below, I’ve revised what I said about the state of the Public Library.]