"Hitwise monitors Internet Service Provider (ISP) networks to capture the usage patterns of the ISPs user base. Hitwise extracts from the partner ISPs networks a list of the websites visited and ranks them according to the number of page impressions viewed."
Data currently available for Australia & New Zealand, with roll outs planned for Hong Kong, Japan, & Korea [South Africa and the UK also].
NFFC - Whilst Hitwise is an excellent resource for information regarding visitors to your site, I personally consider the $900 a year subscription fee is far too much for this service. When they first started in Australia Hitwise originally had 7 day free subscriptions which were quite useful. I recommend Hitwise's profiles and chart facilities.