SE4 Activity Tagger/Linker
This plugin has been tested on Version 4.0.1 - 4.1.4 and works
Active feed simular to facebook. Link/Tag people in by using the @ sign and there name. A dropdown with auto suggestions will appear like facebook and then you click on it to link them into the post.
The user who was linked into the post will then get a notification about being linked to the post with a link to show him the post in question.
Great little feature which makes the activity composer that little more functional!