Google Wave (currently in preview) is a new online collaboration tool created by Google. It provides the opportunity for asynchronous and synchronous communication among users. Waves allow participants to share resources, images, videos, documents, etc in order to discuss, collaborate, etc. It's a wonderful tool and my guess is that it will only become more powerful when it is open to all and a little more polished. Even in it's current state, it's quite robiust.
This is a wonderful resource from Discovery Education. The .pdf file provides you with 30 excellent Web 2.0 tools and a challenge to acompany each one.
ToonDoo is a very user friendly web based application that allows you to create cartoon strips and even cartoon books. The site is designed to encourage participants to create new strips, share their work, and discuss cartooning. What a great way to get students involved in the writing / digital story telling process. The Toon Creator is full of backgrounds, character, props, and other useful tools to aid in creating your masterpieces. I would say that the site is certainly appropriate and easy enough to use for elementary students but is entirely appropriate for older students as well. What a find :)
The Hot Potatoes suite includes six applications, enabling you to create interactive multiple-choice, short-answer, jumbled-sentence, crossword, matching/ordering and gap-fill exercises for the World Wide Web. Hot Potatoes is not freeware, but it is free of charge for those working for publicly-funded non-profit-making educational institutions, who make their pages available on the web.
One of the things that I really like about Hot Potatoes is it's seemless integration with certain Learning Management Systems. I use Moodle quite a bit, and you can include a Hot Potatoes activity right in the Moodle course.
Puzzlemaker is a puzzle generation tool for teachers, students and parents. Create and print customized word search, crossword and math puzzles using your word lists.
Online Organizer - Plan * Organize * Share
Zoho planner is a tool for online to do lists, reminders, notes, and images. Share it with your friends. Get things done - it's simple & free
Includes tutorials on various Web 2.0 tools. The Collaborative Projects listed are most informative. This site also provides examples of using Google Earth for collaborative biology projects, although it is applicable for other disciplines as well.