StumbleUpon.com - Not for bookmark organization. Good discovery engine with an excellent add-on. Attractive site.

Folkd.com - social bookmarking site with an average add-on. every link added to the site requires typing in a Captcha without Pro account.

SecondBrain.com - Social bookmarking site. Decent bookmarklet. Can organize websites into collections with an automatic screenshot.

bagtheweb - a social bookmarking website where a "bag" is filled with associated links. links look attractive. not a lot of activity.

Linkpager allows one to group links into different categories and share them. Screenshots of the webpages can be captured automatically.