nice project man. You "simply" scan the whole blockchain searching for all transactions made to those addresses listed in you site? And then sum the amount of BTC? Right?
Yep, but we add in 50 btc for the genesis block's coinbase transaction (
read more). I'm also planning on adding more to the sum, like btc sent during OP_RETURN, and a few other addresses. The site uses SmartBit.com.au's API, and only updates once every eight hours because the current way we do it uses one API request for each address, but we're changing it soon so it only uses 2-3 requests (200 addresses will be searched in one request) so it will be updated in much shorter (~1 minute) intervals.