Sharing files and documents across computers and devices
Dropbox
When you put your files into your Dropbox folder, they're instantly available on any of your other computers that you've installed Dropbox on (Win, Linux, Mac).
There are also apps for iOS and Android available. The Dropbox app for iOS is able to display pdfs and office documents. You cannot edit them, tough.

You get 2GB for free. 250MB for every friend you invite (8GB limit, though). If you pay, you can get even more space.
Growl integration for sync notification updates.
Web interface.
You can share folders with other people while keeping other folders private.
Create download links to your contents.