CoSketch.com is a free service that allows you to create online sketches and share them with other people, in real time.
You can also save your sketch as an image. The sketch tools include text, colors, lines, curves, shapes, and more.
It is completely online and runs in any browser without the need for plugins.
You can also import images and Google maps images.
This would be great for teachers to use with their students, or for student groups to use while at home for collaboration.