OnLive Desktop, a free app available on iTunes, lets you run virtual versions of Microsoft Word, Excel, and PowerPoint on your iPad. The programs run on remote servers, but given a reasonably fast Internet connection, it feels as if you're working on a desktop. The app has accurate handwriting recognition as well as an onscreen keyboard.
Paid versions add Internet Explorer, Photoshop, and other applications. Free and Plus versions come with 2 gigabytes of cloud storage; Pro gives you 50 GB. Versions for Android devices, iPhones, PCs, and Macs are in the works.
Cost: Free to $4.99 (Plus) or $9.99 (Pro) per month.