I'd recommend
Documents to Go, which has a local directory, and a desktop directory. The desktop one is what you keep in sync with the (free) desktop app, i.e. you choose a folder (My Documents etc.) on your pc/mac which you want to keep in sync with the iPhone app, then with the two on the same wifi network, you can syncronise any files within that directory between the two. Docs2go is great if you also want to be able to edit docs and spreadsheets, and it has a very good PDF viewer, ideal for long ebooks, tech docs etc. The exchange edition of docs2go also let's you view/edit attachments sent to your email account (if you use exchange).
If you don't need the editing functionality, then there's QuickOffice's MobileFiles app which let's you view stuff on your iDisk (great if you're a MobileMe user). There's a host of other iDisk access apps, like Air Sharing Pro, Good Reader, ReaddleDocs, OneDisk etc etc.
My recommendation would be docs2go if you want editing and exchange attachments access, Good Reader for iDisk (or any other WebDAV server). Good Reader also has great PDF viewing features, and you can transfer files over wifi and download files from the web. Basically, get this if you're not bothered about editing.