Photo credit: Jfens

1. Text The Mob: Build polls and message boards for any event
2. BookGoo: Annotate, highlight and share documents with others
3. Kiobo: Discover and share interesting sites with other people in real-time
4. Tell-a-Friend: Let your visitors share your site via email, IM and social networks

1. Text The Mob
   
   Text The Mob is a web based service that you can use to build polls and message boards for any event. After you create a poll or message board, people can start sending responses, feedback and normal text messages via SMS or web and you can then see results in real time. Text The Mob is free to use, premium version is also available.
   http://textthemob.com/






2. BookGoo
   
   BookGoo is an online annotation tool that lets you annotate, highlight and share documents with others. Just upload files in pdf, html, doc, xls, jpg and gif format, annotate any part of it as if you were writing on a paper, and wait for other people to provide comments and feedback. BookGoo is free to use, registration is needed.
   http://www.bookgoo.com/






3. Kiobo
   
   Kiobo is a Firefox toolbar that allows users to discover and share interesting sites with friends or the entire community as a whole. Through a simple interface, you can see in real time what your friends and all the other people are currently browsing, and make filtered researches to find what you are really interested in. Kiobo is free to download and use. Registration is needed.
   http://kiobo.com/






4. Tell-a-Friend
   
   Tell-a-Friend is a Java based script for your blog that users can use to share your website without even leaving it. With a simple HTML code, you can embed a small button that users can click to easily share your website with others via email, IM (GTalk, Yahoo, MSN and AIM) or social network (Facebook, Twitter). The service is completely free to use, with no registration needed.
   http://tellafriend.socialtwist.com/index.jsp