As Beat have requested, here is my respect for the whole plugin API thing and documentation
I have already made three small plugins for personal needs:
-Check user's email domain on login and allow only the specific one (like university email)
-Disable change of name and surname for users
-Automatically create image dir for new users and delete, when user is deleted
BEFORE plugin API I made CB hacks for all these features!
In my plans I have a plugin for automatically adding user to specific Docman group according to registration info; and one for automatically setting some CB attributes like student or teacher (checking this thing on registration in external table).