At the recommendation of another agency we work with, I've just started using Dropbox. And it's brill.
Very simple: register and download, and Dropbox sets up a folder on your computer - a 'box' you can 'drop' work into (inc. as many subfolders as you want).
This then mirrors with the same folder on the Dropbox site.
Install Dropbox on another computer, and you folders/files will automatically sync: make changes on a file at home, and the up-dated doc will appear on your system at work. Neat!
You can also access the web-based folders from anywhere you happen to be working.
And folders are sharable with others, if you're working collaboratively on a project...or just want to show your mum some photos.
There's even an iPhone app, for those who can't do without.
Anyway, sign up here.