If you are following the US election circus, then the best site I have found to the raw opinion poll data is Real Clear Politics. They aggregate all the various national and local polls, and allow you to dive in and find what you want.
For analysis there is of course Nate Silver’s FiveThirtyEight. He is the man who correctly predicted Obama’s election by looking at the actual data, when most other pundits were basing their opinions on what they wanted to happen.
But for my opinion the most accurate predictor of the US election outcome is Paddy Power. There you get the wisdom of crowds, reinforced by people actually putting their money on the outcomes!