The buddylist module allows you to mark other users on the site as "buddies". This is two directional, so you can mark someone as a "buddy" who does not necessarily count you as one.

Buddies are listed in your FOAF file, as well as viewable through your user profile. If enabled, a "My buddy list" and a "My buddies' recent posts" blocks are visible. Recent posts display recent blog posts from your buddies.