Grand Duchy of Luxemburg
I haven't been here yet. But having lived eight years in Brussels, and still not having been to neither Bruges nor Gent (compulsory for even just visiting the country), I realised Luxemburg is pretty close, and somebody whispered in my ear that the trails are pretty neat, too. So now I am rearing to go!
Grand Duchy of Luxemburg is not to be confused with the Luxemburg province of Belgium (covering Bastogne and Houffalize). It is a country of less size than the Belgian province, divided into five provinces, whereof the northern Ardennes province holds all the 15 permanent mountain bike loops (click on Sports). The loops range from 10 to 40 km, in all covering some 300 km.
All routes are supposed to have the same kind of trail signs, and all routes are found on the same map called "Bike Tour" (5 euro ... but if you take it with detailed trail description: 15 euro).
NB. Always try to encourage the effort made: Buy the trail map and help finance mountain biking in the area!
NB. Check the pdf file, it contains some nice information about the small villages, and also contact info for the numerous Tourist Offices.