dropbox account for a few months now, and it's probably the most used web service / piece of software on my laptop. If you're not familiar with Dropbox then let me explain. It's a small piece of software that you load on your PC and it creates a folder, just like any other, that will sync everything you put into it, with a secure service on the Internet. If you have more than 1 PC and you load Dropbox onto them all, then those files will be in sync across all your computers. Although maybe not its primary use, it's a really easy way to copy files between computers at home or home and work. It also acts as a secure way to backup any important files, and to share files between other people who may or may not have a Dropbox account. I use it every day to copy files between computers and to share individual files that are too big to email.
To start with, you get 2GB of storage for free, but you can earn more by referring friends. Each one you refer gives you and them, and extra 250MB. If you want to sign-up, use this link to get yourself (and me!) and extra 250MB right from the start.
For the geeks like me out there, there are also apps for Android and iPhone devices that makes copying files off your phone to your PC (photos and music for example) really easy.
Update: Because I run a server at home, I'm now running Dropbox as a service on that server. It means that my files in Dropbox will sync to my server, even if I'm not logged in. Neat. I got help from this post.