Twiddla is an Online Whiteboard...and it is super simple! Users can mark up websites or graphics, or start drawing on a blank canvas. It is a really great tool for distance learning led by a teacher or peer to peer. It even has a built in Equation editor for the mathletes out there.
Twiddla requires no plug-ins or downloads and works on any browser. Best of all - NO account required! This is great to maintain student privacy. (The free version allows for up to 10 participants to meet for 20 minutes. There are other pricing structures.)
To get started, start a board, and share the link.