GoodReader. Super-robust PDF reader. Syncs with Google Docs, MobileMe, Dropbox and more. Free download for iOS.
Kindle. Access and read your Amazon Kindle books even if you don't have a Kindle. Free download for iOS and Andriod.
Papers. Organize, read, annotate, and share your collection of articles and PDFs on iPhone and iPad. Free download for iOS.
Documents to Go. Free downloads for iOS and Andriod. View, edit, create, & manage Microsoft® Office files on your mobile devices.