New and free cloud computing service soonr allows you to designate a folder on you pc or mac which will then be available to you on your iphone or (when wirelessly connected to the internet) on your ipod touch. Its great for putting papers and presentations and maps and such onto your iphone, but unlike the (not so free) AirShare app it doesn't work when offline (sry iPod Touch users).
Anyway, to get things running, install the soonr app from the app store and setup an account through it. Then you will be emailed instructions on how to install the client software on you pc or mac. Careful though, by default it want to also backup all the files in your my documents directory, but you can turn this off during the setup.
UPDATE: So it turns out that dropbox has a reasonable mobile interface accessible through safari...not as nice as soonr's app, but just as effective. Not only that dropbox is fully cross platform supporting Linux, MacOSX, and Windows file syncing. So I am rating it the more powerful utility.