The best iPad utilities most everyone should have The iPad can't open Zip files -- an amazing omission in the iPhone and iPod Touch as well. There are several apps that can unzip files, but the best is ZipThat ($1). Its clean interface makes it easy to use.
Although the iPhone comes with a calculator app, the iPad doesn't. There are several calculator apps for the iPad, but I prefer the simple, capable Calculator Pro ($1). If you do lots of calculations and want a tape function to capture all calculations (and email that history as a file), then get the less aesthetically pleasing Calculator HD ($1).
If you view Photoshop native files, such as for page layout, Web, or presentation projects, get the AirFilesHD app ($1), which also offers Wi-Fi file sharing and basic drawing capabilities. As you can see, Wi-Fi file sharing is built into lots of apps!