Keynote – presentation app that isn’t as flexible as PowerPoint, but has very cool transitions and templates like chalkboard.
Socrative – this is a free clicker app. Technically it’s an iPhone app, but works well on the iPad. The Teacher app has a companion Student app. Students can use their smart phones, iPad, or regular computer with online access to answer questions.
ShowMe – a whiteboard on your iPad when you don’t have a whiteboard or a chalkboard or chalk.
RDM+ – when you forget a file that’s on the desktop in your office, this app gives you access to your “other” computer. (Saved me more than once.)