Is there a way to identify whether users are online or not? Something like online offline status symbols used in a chat application for every user.
I think this is an interesting idea, but would be concerned about the overall number of users on the site at any given time cluttering it up. Maybe a count would be sufficient with the ability to drill into that number to see the user names?
This could also tie in well with the idea of a friends list where an icon could be added next to their user name if online or something along those lines (think Office Communicator type of thing)...