Polleverywhere allows you to create questions and have people respond via text message, Twitter, or web. Great for use in a computer lab. Could also be used in conjunction with hand held devices (iPod touch, iPad, etc.) as a student response system.
Glogster is a free tool students can use to create media-rich online posters. Students have the ability to embed sounds, videos, pictures, animations, and graphics into their glogs. The http://edu.glogster.com domain is separate from the rest of the site, so the content is appropriate for schools.