Ryan Imel

This plugin adds the ability for users to “check in” to the P2 theme when they’re active. Once activated you’ll find a new “Who is Checked In” widget that you can add to your sidebar, and a “Log In/I’m here!/I’m leaving!” button will automatically be added to your P2 theme’s header.
The sidebar will list the users who are currently checked in, along with how long they have been checked in. By default users will be bumped to the bottom of the list and be shown as checked out for 30 minutes after they have left.
The language in this plugin is purposefully left a big vague so you can use it for your own purposes. Checking in could mean literally checking into a building to show you’re there (the purpose I developed the plugin for) or it could mean that you are simply available, and actively watching the P2.
In addition, the total amount of time someone has logged when “checked in” is displayed underneath a user’s name in the sidebar.
If a user ever forgets to check out, admin users (or those that can manage_options) can click the x button next to their name and check them out for them.
<h4>Kudos</h4>
Props to the Who’s Online plugin for P2, since I cribbed a number of the elements from that plugin to make the development of mine go a bit faster.
Also, the button styles are based on the Twitter Bootstrap button’s styles.

P2 Check In

This plugin adds the ability for users to "check in" to the P2 theme when they're active.

fdqps

‘Save to foursquare’ Button for your blog posts. 
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‘Save to foursquare’ Button for your blog posts.

P2 Check In vs Save to foursquare