I may be wrong, but the only apps I've seen that do that work over wifi, not USB. Which is great at home, but then you're out of luck at the library computer if you're trying to get the files back off the phone!
Here's a better idea. Get Dropbox (2 GB is free, you can pay for more.)
http://www.getdropbox.com
You have several ways to use it:
1) When you instal it on your Mac it creates a folder on your Mac that syncs with your online folder. Put something in that folder and it's suddenly online.
2) Make a home-screen bookmark to the website on your iPhone. It's formated for the iPhone so it's kind of like having an app. You can view your files on your phone that way.
3) Go to their website on public computers, log in, and download or upload any files you need, then log out.
You can also put files in the 'Public' folder, right-click them on your Mac, and select the "Dropbox" menu option to get the URL for sharing the file with people that way.
I've been very happy with it.