EuroVelo, the European bicycle route network, is a project of the European Cyclists' Federation. Much of this text has been lifted from their web site. EuroVelo is made up of 12 routes, totaling over 66,000 km, of which about 45,000 km is already in place.
The EuroVelo routes are made up of existing and planned bicycle routes at a regional and national level. Their web site includes implementation guidelines for on-road and greenway routes. There are also guidelines for signage. Examples abound!
Some of the routes have their own web sites. Route 6, the Atlantic-Black Sea (Schwarzes Meer) route has an interactive map. If you click a section of the route... well, nothing happens! But hopefully they are working on details about each section of the route.