(originally posted 10/11/09)
Scribblar is an online meeting room / white board. Students can use it to collaborate on projects, teachers can tutor via the computer, and teachers can use it for collaborative meetings and projects.
Scribblar gives the users a virtual white board on the screen and writing tools to create text, lines and shapes. They can even add images.
There is also a chat window on the side bar so that users can talk to each other while working. There is even a live chat system if you have a microphone and speakers.
I see students using this to work on homework problems together. Teachers could use this to help students with homework after school or on the weekends.
Dweeber is another resource that has some similar functionality for students.
UPDATE: Scribblar can now support PDF files. Free users can upload PDFs with up to 5 pages. Your uploaded PDF will be turned into an editable image in Scribblar. Thanks to Free Technology for Teachers for the tip!