Few moths ago I also found a site that looks like a directory of btc address private key which could be import to wallet, I try to pick random wallet keys there around 50-70 address out of thousands listed on the said site but no wallet has balance on them off course the one who listed the directory maybe had already withdraw all btc on those address I just think..lol
Is this the site you are talking about:
http://directory.io/If so that is simply a web page that calculates a group of sequential private keys based on the "page number" and then calculates the two possible Bitcoin addresses for each of the private keys on the page.
Nothing special or new there.