This plugin will show all users currently online, followed by all offline users in italics. You can click on a user's name to view all posts by that user, and if you hover the mouse over a user's name it will show you how long ago they were online. You need javascript enabled for the list to update without refreshing the page.

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Source:→ wp-plugins.net

AJAXed Who's Online, Plugins, WordPress Plugins, WordPress