Diigo is my number one choice for collaborative bookmarking. Students can create groups or you can create groups for them to which they contribute bookmarks. Bookmarks can be annotated with notes about what makes that bookmark useful for the project that the students are working on. My friend Mary Beth Hertz wrote a nice overview of Diigo a couple of years ago, you can read that post here. The video below provides an overview of Diigo's features.
Diigo V5: Collect and Highlight, Then Remember! from diigobuzz on Vimeo.
Think Binder is a website that gives students a place to create online study groups. In each group students can share files, share links, chat, and draw on a collaborative whiteboard. Students can create and join multiple groups. As you will see in the video below, getting started with Think Binder takes just a minute.
Embedded below is my brief video overview of Think Binder.
Searcheeze has been shutdown.
The video below offers a short overview of Searcheeze.
Searcheeze - Search collaboration made easy! from Searcheeze on Vimeo.