I heard about the Belgium Ardennes. It seems a natural area with very nice villages and numerous outdoor activities. Is there a website or resource that lists them?
I am particulary interested in hiking but any other activities are welcome.
Basically you are asking about the outdoor activities in a region spanning half a country, where outdoor activities are the main tourist attraction. That is rather broad and you certainly won't find an exhaustive list.
A starting point would be the Commissariat Général Au Tourisme of Wallonia: http://www.tourismewallonie.be. The site is in French and I don't find a functioning English version, which appears a bit strange. Outdoor activities can be found under "Sport, détente et sensations" and there is a section specific about hiking (Randonnées pédestres). That is a good starting point.
An alternative is the tourism website of Wallonia-Brussels: http://www.opt.be/ There is a section "Nature & Sport", under which you find information about "Treks and walks".
Both of those website cover at least Wallonia. The Belgian Ardennes are part of Wallonia, so you will have to discard part of what you find, but almost all of the outdoor activities will be located in the Ardennes.
Once you have decided where you would like to go, I advise you to check the website of the local community as well. Most have a tourism section covering this kind of activities.
You specifically asked about the Belgian Ardennes, but for completeness I add a link to the tourism site of the French Ardennes: http://gb.ardennes.com/ The French version has a link to hiking and mountain biking circuits that leads to this page.
I think this is what you are looking for : http://en.ardennes-etape.be/tourism-in-the-ardennes. I have used the site mainly in the past to book cottages, but it contains a lot of information about activities.