I was recently exposed to another free and great teaching tool.
http://www.polleverywhere.com Polleverwhere allows for real-time polls to be conducted anywhere. Users can vote via email, web voting or even text message. This allows for the person conducting the poll to gain instant feedback from their audience. Imagine harnessing the power of cell phones as an educational tool in the classroom. This could be an alternative to student response systems, perhaps at the college level instructors could use this as a way to quickly take an anonymous poll.
The website also has a feature that allows messages to be sent to a wall. This could be potential beneficial in the classroom. However, all messages are unfiltered and unmoderated opening the door for a digital public stoning. With all web 2.0 tools the there are some struggles with striking a balance between collaboration and privacy.