Activity Streams extensions

One document we're supposed to keep up as a group is the Activity 
Streams extensions page on the W3C wiki. This is a list of extensions we 
know of that are used with AS2 documents. From the AS2 documentation:

/TheSocial Web Incubator Community Group 
<https://www.w3.org/community/swicg/>maintains a wiki page onActivity 
Streams extensions <https://www.w3.org/wiki/Activity_Streams_extensions>./

In the course of working on the ActivityPub Miscellaneous Terms 
documentation, I cleaned up the wiki page and added about a dozen 
extensions. I also intend to go through the FEPs to find others, as well 
as add per-implementation namespaces as I find them.

https://www.w3.org/wiki/Activity_Streams_extensions

However, this is a group-maintained document, and a wiki page, so if you 
know of an extension that's widely used, please feel free to include it.

Evan