Random ideas, probably nowhere near complete:
* [WMS Racer](http://wiki.openstreetmap.org/wiki/JOSM/Plugins/WMS_Racer) Plugin for JOSM
* Skobbler's (announced) [Address Hunter](http://oegeo.wordpress.com/2011/06/06/openstreetmap-usability-converting-more-sign-ups-to-active-contributors/)
* HP Mediascape (talk by Ben Clayton at [SOTM 2008](http://wiki.openstreetmap.org/wiki/State_Of_The_Map_2008))
* text-adventure-like game on OSM (slightly outdated, [article on mailing list from 2007](http://www.mail-archive.com/newbies@openstreetmap.org/msg00419.html))
* [Monopoly City Streets](http://wiki.openstreetmap.org/wiki/Monopoly_City_Streets)
* [Nike Grid](http://blog.oobrien.com/2010/04/nike-grid/)
* lots of flight simulators using OSM