Addresses can be grouped as belonging to the same user (or, better, same "account") by grouping together the addresses that participate as inputs to a single transaction, and doing it transitively. This has been done in the recent paper by Shamir and Ron, by some previous researchers and can be done by znort's parser. I know this is not 100 % accurate but I'm fine with it.
What I'm looking for is a site that displays the results of such grouping graphically, possibly with transactions between accounts as well.
I've read about such a site on bitcointalk.org, but I can't find the topic any more. I'm looking for the url of the (now possibly defunct) site or the bitcointalk topic describing it.