Dropbox creates a folder on your computer, which is continuously synchronised with a personal, secure space on the web. The system gives you 2GB of online storage for free, and you can purchase more as required. This is a great way to share files online, either to the public in general, or to a selected audience. The desktop integration makes it extremely quick to use, and the fact that it works on all three major operating systems (Linux, Mac, Windows) means it can be used by all.
If you have ever found it impossible to email a large attachment, Dropbox provides a great solution.