ShowMe Interactive Whiteboard' app would have to be one of the most useful apps in the classroom for both teachers and students.
When I am demonstrating it to teachers I get them to show me a maths strategy they are teaching their students at the moment.
They record their voices talking through the strategy and draw it on ShowMe's Whiteboard.
The final 'ShowMe' can be uploaded to http://www.showme.com where you can then get the 'embed' code to embed into your wiki, blog or site for students to revisit.
Once the strategy is taught to the students they can then be set similar problems to solve.
They can record their voices explaining how they solved the problems as they write the solution. These can then be shown to the rest of the group or to the teacher. These could be assessment examples that students can add to e-Portfolios.
Have a look at this example from Russell Street School in Palmerston North