Air Sketch is a phenomenal app for use in the classroom if you lecture with power point slides. It allows you to freely walk around the classroom with only your ipad in tow, while still having full control over what the students see projected. Even better, it allows you to free-draw directly on the slides so that you can annotate graphs or draw student attention to something important.
Air Sketch is $9.99 in the app store. I personally used it in conjunction with pdf versions of my slides (which is what I've always use since I prefer creating slides in Keynote rather than PowerPoint). You simply put your file in dropbox or email the the file to your ipad and then "open in" Airs Sketch. From there you click on the icon for "server settings" on the bottom left-hand corner and up will pop a URL.
The magical part comes next. Simply go over to the standard in-class computer and type this URL into any web browser. Click the settings on the tower to project the computer (not the ipad, which is not wired to anything) and presto! your ipad will now be projected onto the screen! You can now flip through your slides and draw on your slides completely wirelessly. It's pretty cool.