Microsoft has at last released an iPad version of Office however since it has taken a while to actually get to the iPad, many competitors have popped up. There are now some fantastic alternatives to Office for iPad that make the iPad into a pretty good work machine.
Apple's Pages, Keynote and Numbers
The most obvious are of course Apple's selection of Pages, Keynote and Numbers. Apple has had the apps for a few years now. You did have to pay for them when they were first released but now they come free with new iPads and Macs.
They're fantastically adapted to the iPad and work great with iCloud for syncing across platforms. They're probably the best out there right now for the iPad, iPhone and Mac.
When launching Google Drive, Google integrated Google Docs into the platform allowing people to edit word documents, spreadsheets and presentations.
If you're a Google user this is a no brainer really. Google Drive works across nearly every major platform and like Apple's apps, it even has a web interface.
A unique feature of Google Drive is that you can invite other people into documents too so you can easily collaborate.