We made a plugin for FF, which measures the time spent on the pages, and then
1) considers the personal top sites
2) considers a common top sites
http://feedme.ru/3) anonymously compares profiles and recommends new links to the user based on the common interest in other URLs - you can subscribe to the news on RSS
(invite at the end of the post)

It is clear that this is somewhat similar to StumbleUpon and other systems of social recommendations - but we are doing a zero-effort system: the user should not do anything except install the plug-in. I will still receive recommendations.
Although if he wants to set up the system more precisely, he can recommend urls and filter secret urls and clean the history already sent to the server.
')

By default, the plugin sends data to the server automatically, but you can turn it off and check all URLs manually before sending.
Here a little more detail -
http://feedme.ru/about.phpHelp, please, with testing - it is clear that for the work of social recommendations you need a statistically significant number of users. Invites code - eguinyh0jlb2ov1w
Registration -
http://feedme.ru/signup.php