Do you mean for dating or discussion
This one, poly percolations and fetlife are the more active poly sites. Fetlife being for pervs, but the poly community is active
for dating, okcupid is the resounding "best"
there are also *shudders*.. some news groups, talk groups on yahoo. If you can stomach the horribleness that is yahoo groups, you could search through them and find local ones that can connect you to other poly peeps
lastly some of the larger cities use meetups.com to list their poly meetings.