Collaborize Classroom: a free collaborization tool for teachers and students. The teacher or students can start discussion questions, add video or picture attatchments, and create polls, multiple-choice questions, or specific task groups. Free to use.
Conceptboard: an online non-flash collaboration tool. Users can collaborate on a photo, presentation, or any other project simultaneously in real-time. There is a free basic service (allows for 20 boards), other plans vary in price.
Glogster: create a collaborative online poster using images and graphics from around the web. Free to use.
GoogleDocs: use the same types of programs found in Microsoft Office including a word processor, spreadsheet, slideshow, and others. Users can share their documents with other GoogleDocs members and collaborate on a document simultaneously in real-time. Documents are saved online, so they can be accessed from anywhere. A must-see tool!
PBworks: a collaborative website building tool. Teachers can set access rights for students in their classrooms to upload to or modify the class website. There is a free basic service, but the classroom edition is $99/yr.
Poll Everywhere: allows you to create an online poll where participants text in their votes via their mobile phones. The service is free for up to 40 participants, different rates apply for additional participants.
Study Hall App: create an online "study hall" session. The teacher can upload presentation slides before class begins, post questions on an interactive whiteboard, post comments on uploaded material, and more. Free to use.
VoiceThread: an online collaboration tool. Users can comment on a presentation with text posts, audio or video recordings, or with many media formats. Conversations are saved in the website's database for future viewing. There is a limited free version, other rates vary.
Wallwisher: an online, collaborative bulletin board. Free to use.