It depends on what you're looking for. As has been mentioned, artofproblemsolving.com (aka AoPS) and mathlinks.ro are good for the contest types. I've taught an

It depends on what you're looking for. As has been mentioned, artofproblemsolving.com (aka AoPS) and mathlinks.ro are good for the contest types. I've taught and graded for AoPS in the past, and there is no other place that can compare for contest math. On the other hand, the overall skill level at AoPS is very high and very contest math oriented. I don't really check it that often anymore but in the past it never struck me as discussion forum as much as a problem solving forum (or a "how to become a better problem solver" forum). In addition, the majority of the posters there are young, that is high school age and below (though there are still a fair number of older posters). To put it a different way, when friends of mine that are interested in learning "interesting math" from a casual standpoint ask me where to look, I'm not really sure what to tell them. Certainly I recommend giving AoPS a shot, but I'm unsure as to whether it would be the best place to go.