With a Pro subscription, the CloudOn app nicely accesses the 2010 versions of Excel, PowerPoint, and Word, as well as Adobe Reader, hosted on Windows Server. Plus, it supports the iPad's native keyboard and Share facility. It uses your Box, Dropbox, Google Drive, and SkyDrive cloud storage, so you don't have to worry about version control across devices.
It has a few flaws, but when you need the full version of Microsoft Office, CloudOn is a handy option -- as long as you have a live Internet connection, of course. The free CloudOn service remains available but lacks essential business features, such as revisions tracking.
Price: $80/year (Pro subscription)
Compatibility: iPad and iPhone