Searched on this but couldn't find anything about this. Kinda strange this isn't developed by someone yet.
Registrated users can make a friends list. This list will be displayed in the sidebar. When a user adds a friend (by clicking a name or type it in manualy in the sidebar), it will add the person to the friends list. You can show ONLINE friends (friends browsing the the site) and offline friends (with an option to collapse this offline list). The pluging uses AJAX to keep up to date (with who is offline and not). Well this is kinda the idea!
Hope someone can do this :)