This is the granddaddy of online apps. Besides providing a way to have your documents available wherever you have an Internet connection, you can also share your documents with other users. There are downsides to this of course, but in general it works very well, and is an acceptable solution for classroom use (read the Google legal agreement though). The apps have a pretty rich feature set.
This is very impressive. The author has assembled a site of online collaboration mind-mapping tools.
A kind of voicethread for sketches. You can sketch something, add voiceover, and share with others.
Online whiteboard for drawing & team collaboration - Interactive whiteboard software
Free online whiteboard collaboration tool. Has added advantage that there is an iPad app. Free for up to five collaborators. Includes a chat feature.
Online interactive collaborative whiteboard. Has a grid option, so it's usable for doing graphs.