Really simple online visitors counter using google appengine's memcache. It was build to use with "SiteApps.com - http://siteapps.com/app/online_visitor_counter-238"
Search for the app "Online Visitor Counter"
It uses an uniq ID to identify users on a dict and expire the visit after a pre-configured time.
It should run in conjunction a simple java script that need to care about:
-An uniq ID to the visitor -An uniq ID for the website -Set the cookie with the uniq visitor's ID -A function to show the online counter