While I am known in my district as "that podcast guy" the Web 2.0 tools I like the most are wikis. The ability to create a web page quickly that allows others to edit is like a dream come true for teachers who want to do collaborative projects but not sure how to spread out the credit for participation in completing the project. There is always one or two groups where one person does most of the work while the rest sit back and take credit. Good wikis allow people to see a history of a page's development with who has done particular edits to a page.
PBWiki has become popular because of its ease of use and its word processor-like wysiwyg interface. Now there is a new wiki for the more random, left-brain, creative types: PikiWiki. PikiWiki is like doing a scrapbook, you start with a blank sheet then paste text, pictures, video, audio anywhere on the page. Each item on the page is like a note. If you don't like the look of your page you can slide the notes around to suit you. There are also tools to help you with the alignment of your items. The concept is good and should prove popular to teachers who embrace wikis as classroom tools. Click here for a sample page I put together using PikiWiki.
Keep in mind, PikiWiki is in a public alpha which means this wiki site is just getting started and still has bugs to work out. One problem is the site does not show the page history which shows page comparisons detailing the evolution of a page. Since you cannot see the page comparisons, you cannot restore an earlier version of the page if someone attempts to sabotage the page (we all know students who might try this). Once the kinks are worked out of the system, the people of the PikiWiki told me they will, PikiWiki should become a great tool for teachers and students.