Five sites that should support OpenID

• Listal
• passportstamp.com
• Last.fm
• OpenStreetMap (both their wiki and the map editing itself)
• Twitter

OpenStreetMap and Last.fm both pose an interesting problem in that as well as a web application they also use desktop apps. In OpenStreetMap's case their editors like JOSM need to authenticate against the REST-based API, and in Last.fm's case the Audioscrobbler plugin needs to authenticate with the server when it reports what tracks you're listening to. That something is hard doesn't mean it's not worth doing, though!