Sometimes, the best way to explain something is with pictures. Other times, you’re trying to show someone math over something like IM, and failing miserably. In both cases an online whiteboard application comes in handy.
The most recent one I’ve found is Dabbleboard. Not only can you draw, but you can insert text just be clicking and typing, insert math symbols, maintain a library of symbols you use often (quick and dirty schematics anyone?), insert PDF and other types of documents, and images from either your hard drive or a URL. Downsides: no flood fills, limited colors, no spray paint, and the automatic shape detection can be buggy (but you can always go freehand).
Another one I’ve used is the GE Imagination Cubed, which has the advantage of being able to use the pens as pointers, but generally lacks a lot of the cool features Dabbleboard has, and is more difficult for inserting text.
I wonder if Google Wave has any good whiteboard applications yet… Even if it does, Dabbleboard and GE Imagination Cubed offer the advantage of just having to share a link, without the hassle of login names and registrations before your group can get started scribbling.