I've been using dropbox for quite a while now. Basically dropbox is a service that allows you to share files over the internet. And it gives you 2GB of space for free! There is a desktop client for Mac, Linux, and Windows.
I initially used it to share single pictures and files to friends via the internet using its 'public link' feature. Basically this allows you to give anyone on the internet a link to the file, without needing them to sign up for dropbox. There are a lot of other sites out there that allow you to do this, but dropbox has the advantage of linking straight to the file, and not linking to a page that can let you download the file
More recently I have been using dropbox as a shared folder for group projects where there are several people working on a collection of files at once. It even sorts out file collisions as well!
Here is an exciting invite link that will give you an extra 250mb of free space: http://www.getdropbox.com/