Users Online 1.1
The amount of time can be configured in the plugin's settings. Users are tracked by ip address, so both logged in users and guests are included in the total.
Text can be displayed after the user amount total (e.q. "10 users browsing") and is configurable from the settings.
The total is displayed at the bottom of Categories/Discussions (thread list) pages but this can be changed based on your own preference by changing the event hooks (Render_Before, AfterRenderAsset_Handler, etc) in the code.