Up until a few weeks ago, photo.net was the best general photography forum in the world. That has, unfortunately, changed. (Write to me if you want the long explanation of why you should avoid photo.net.)
Since photo.net went downhill, many of us ex-photo.netters have been looking for a good alternative. The general consensus is that the best alternative is photosig.com. However, I've heard that, while it may be the best alternative, it is still not that great. I've yet to try it.
There is also photocritique.com. It has good points and bad points.
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Are you looking to get your pictures critiqued? Looking for equipment reviews? Hot tips on photo locations? I might be able to give you better leads if you tell me precisely what sort of forum you are looking for.