in my opinion, the absolute best (and now, in newer versions of the ubb, its a standard) is the "who's online" mod.
it proves itself again and again as a great way to stimulate conversations.
when users are able to see that their friends or "debating enemies" are online, they are more apt to add to a specific conversation, in hopes of engaging in a discussion with the other person. -- excellent way to make your current members wanna stick around longer, and be more active in general.
