Important note: I am not responsible for the contents of external links. Please use the links only if you are in agreement with this condition.
In Germany there are 2 important collector's associations, ibv and FvB:

Link to FvB:   FvB-BDM

Very much detail about German breweries and their history can be found with Klaus Ehm. The emphasis lies on labels, but also for beermat collectors the site is highly interesting.
Dieter Beck from Middle-Franconia offers on his homepage
a lot of interesting information for collectors as well as pictures of international beermats. The site contains many links to collectors.