Quite often the same model can be found on different sites at the same time. Sites provide different options and different communication channels to the users.
Probably the most popular in Belgium is http://modellennet.be. Currently it lists more than 2000 models (mostly from Belgium and some from the Netherlands). In Dutch.
Another popular site is http://www.eye-on.net (former photograaf.be). Current list is above 800 models. The site is in Dutch. It is supposed to grow beyond a basic meeting place for photographers and models, but these services are yet to be developed.
Less popular is http://models.handelsgids.be/?zoekenHome/
Speaking about Dutch model the most popular site is http://DutchHeaven.nl
Very popular international site: http://www.modelmayhem.com. It is not so popular in Belgium (only 300 models) as it is in some other countries, but is it a good resource for photographers. The site is in English.
Site popular in Germany: http://www.model-kartei.de. It lists more than 20,000 models, and on top many make-up artists, studios for hire, etc. Although the site is in German it is quite easy to search for models if you learn a few basic words.