bookmarkg.com - social bookmarking site. sites have to be entered manually. links can only be shared with those that you personally invite.
yourversion.com - another social bookmarking site. decent; good bookmarklet.
Blinklist - The Blinklist toolbar & Google Gears doesn't work with Firefox 4; My experience with Blinklist was quite unsatisfactory.
Sharetops.com - A social bookmarking platform where users can make customizable web desktops to launch various links, games, & more.
Diigo.com - Free account allow for unlimited bookmarks but has limitations on some of its other features. The Firefox add-on is excellent.
Sitemark.com - social bookmarking website with a nice layout. visually appealing. links can be put into folders, rated, and tagged.
Flinkin.com - a social bookmarking site where links can be stored into folders using a bookmarklet. link titles can't be edited. decent.
markme.me - a social bookmarking site. a basic bookmarklet dumps pages into your own page.
StumbleUpon.com - Not for bookmark organization. Good discovery engine with an excellent add-on. Attractive site.