If you don’t already have a Board Game Geek account, follow the Homepage link and set one up. Spend a few hours (or days) perusing the site and setting up some subscriptions to content that interests you. Then come back here. Board Game Geek was not the first resource featured here solely because I assume you already know everything you need about it.
Scott Alden and Derk Solko started the site in 2000 that has been growing at an increasing rate ever since. For more information, read the Wikipedia Article or visit the site and find out for yourself.
Often knocked for its dated design and cluttered interface, with almost 75,000 games in its database (Dec. 20, 2014) “BGG” is still the essential resource for board gamers. This is not an intuitive site to traverse and it has so much content it is overwhelming, but get over it and learn how to get what you want from the site and how you can contribute to the dialog and information provided.
Of primary interest to game designers are the game design forums. These are very active, providing opportunities for feedback, contests, access to playtesters, etc. At this point I am watching these forums, but I am not highly active in them. There is so much there and they are so active, it may take a while to become knowledgeable and comfortable enough to know how best to participate. In the end, it is probably a matter of diving in, discovering as you go, and making some contacts that you trust to provide good advice and feedback.
Here are some links to keep up with this resource: