Twitter Bookmarks - Create shortcuts to your favorite users/tweets on Twitter
Bookmark It - Create awesome notes on YouTube videos
TweepsBook - A better bookmarking tool for power Twitter users that you can use to filter, organize and bookmark tweets on your liking.
Bookmark OS - Bookmark OS is like Mac or Windows optimized for bookmarks.
Tweetflick - Save, organize & find Tweets to get the most out of Twitter
PageMarker - Save, organize, annotate and revisit your bookmarks