Is there a product I can buy that would allow to present to the user all sites he is subscribed to? Some kind of directory that would help the users finding their way in all the sites they're registered to.

Is this a SharePoint question? If so, what version of SharePoint? SharePoint 2007 allows the user to see what alerts he's set-up on what lists/libraries, which essentially covers your stated aim above.Steven Andrews | SharePoint Professional | http://www.twitter.com/backpackerd00d | https://baron72.wordpress.com/