BlogPress (Free) is an app that allows you to write posts for blogs such as global2 and Edublogs.
Dropbox (Free) is a really useful free app for teachers, as it allows you to share files between different devices. You can also create shared folders that students can access, so if you would like to distribute resources, students can access the dropbox shared folder. You can also submit and collect work using dropbox. Easily installed on iP*ds, PC's and Macs.

Easy Portfolio, by Jarrod Robinson (the @pegeek) allows you to save videos, images, audio, notes, weblinks and documents for each of your students. This is a great tool to use at parent-teacher interviews to show parents the work that your students have completed during the year.
Explain Everything($2.99) allows the user to record short screencasts with audio. Ask your students to explain a maths problem or a science concept. This app is similar to "Show Me", with coloured drawing tools and audio recording.
"Skitch" (Free) is an app that you can use to annotate images - adding text, arrows, diagrams, shapes and highlights. Great for anatomy (circulatory system, flowers, brain).
"Show Me" (Free) allows the user to record video, diagrams, drawings and audio and share the product on Facebook or Twitter.
"Thinglink" (Free) is a site where you can create interactive images - you create a 'hotspot' on an image you have uploaded and you can link webpages, YouTube videos and any other URL to the image. You could add an image of a wind turbine, a combustion engine or a motorbike for example, and link it to pages that explain how it works, where it is made etc