>> del.icio.us - http://del.icio.us/
This social bookmarking site is becoming something of a web phenomenon. In addition to letting you preserve your bookmarks in an online repository and share them with others, del.icio.us also lets you feed your bookmarks as RSS. It's also fascinating and informative to watch the "most active" and "most popular" lists as others post.
>> Spurl.net - http://www.spurl.net/
Spurl is del.icio.us with a more refined UI and marketing. In fact, in addition to the same types of features that del.icio.us offers, Spurl allows you to share your spurl'ed sites with your del.icio.us account.