Dropbox, I end up using it a lot.
The fact that you can "star" things to keep access to them when you're offline is a great feature, makes it great for travel.
I use iCloud to some extent with Pages, Numbers, and Keynote. They work very well.
There are some reference materials I clip to Evernote as well. Premium accounts let you store entire notebooks offline, which is handy when traveling.
Goodreader is also really useful.
I do ~a lot~ of work on my iPad. It has basically replaced my laptop for all of my mobile computing.