del.icio.us - social bookmarks
<http://del.icio.us/>

It allows you to easily add sites you like to your personal
collection of links, to categorize those sites with keywords, and
to share your collection not only between your own browsers and
machines, but also with others.

サイトを簡単に追加できるし、キーワードでカテゴリ分類できるし、
他の人とも共有できるとのこと。