Attention, new Link: https://ahorn.lima-city.de/tr/ (map.comlu.com is down)
This map visualizes turn restrictions and indicates errors and warnings. It is intended to guide you in increasing OpenStreetMap's routing quality. The map isn't perfect but it is stable enough for the most of us ;)
There are problems in Internet Explorer prohibiting the loading of restriction data for some browser versions. I checked IE 9 and IE 10, they work for me. Firefox, Chrome, Safari work fine.
Turn restrictions are fetched from Overpass-API, which is updated every few minutes.
Wiki, turn restrictions: http://wiki.openstreetmap.org/wiki/Relation:restriction
Wiki, Overpass-API: http://wiki.openstreetmap.org/wiki/Overpass_API