Scribblemaps lets you scribble over the top of google maps or google satellite images. This is excellent for teaching students about mapping, following routes or even working out co-ordinates on a map. This could be used in Maths, Geography, History, documenting Field Trips or even Art. This is easy to use and doesn't even require a login if you don't want to.
Combine this tool with Jing (see previous post) to cut and paste screen shots of the maps that you have annotated, or print out a specific routes travelled on a journey to include in assignments. Good simple Web 2 tool that is easy to combine with other tools for students to further enhance understanding.