Have you ever wanted to have the same file on your work computer and your home computer (or your smart phone/laptop/netbook)? Or maybe you and a coworker need to share some files temporarily. In either case there are a couple services that might be useful to you: DropBox and SugarSync. Both do essentially the same thing. They both provide you with an account online that stores files. That is nice but not an amazing service. Where these services do become useful is when you install a small program on your computer. Once installed it automatically uploads files placed in certain folder to your account. If another computer has the program installed it will automatically download the file.
You can also access your files from any computer by logging onto their website a computer or smart phone.
Both have a free version capped at 2GB, though you can pay for more space. They also offer slightly different features so you may wish to check which one suits you better (my favorite is Dropbox).