We are starting to build TeamCity plugin repository. Well, "repository" is too big a word for it at this time, but more entries are under way.
The idea is not only to list a plugin, but provide plugin documentation space, host plugin repository and bug tracker, and (the most interesting part) to build the plugin against the latest released/current trunk TeamCity versions.
We've set this up for several our plugins (build with TeamCity trunk version is a pending task), and plan to provide the same services to the third-party plugins.
So if you want to write a plugin, or already have one that can be of interest to other users, just let us know.