Online/offline status

Is there anyway you could show when a member is online or offline? I was just thinking it would be handy, say if you're asking someone a question, or answering someone's question on a forum, you could know if they're still online or not. Or you could know when your friends are online. 

what a cool idea- like on facebook... so that the function can be turned off if the user doesn't want to be disturbed...

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