Dropbox is a simple program that allows people to share files just by dropping them in the dropbox folder that you specify. It is similar to a subversion repository except for it will automatically update whenever something is added or removed from the folder. This might not be ideal for all situations but for people who like sharing a lot of files between each other or a small group of people it is pretty nice. Also because your files are stored on the dropbox server you can not only get your files back if your computer breaks down but you can download them from the dropbox website from any computer directly from there website.
Dropbox is very easy to use, simply download the program and create a folder for your dropbox. A blue box will appear in your system tools showing you that it is running. You can create folders within dropbox to share to your friends just by inviting them via email. You are also given a public folder, this folder is very convenient, so lets say you want to share a document with someone who doesn't have dropbox. All you have to do is put the item in your public folder, right click the file and select "copy public link". This is a direct link to download this file just paste that to him or her and they can download it instantly without any secondary downloads.
Dropbox free but you are limited to a certain amount of space, at first it is 2 gigs but you can increase this by inviting friends, up to 8 gigs. Files are uploaded / downloaded to and from the server pretty quickly and unless your uploading a lot of stuff your friends will see it pretty much instantly. I recommend this program for groups of people who share files with each other, such as pictures or documents, small teams that use dropbox for specific things such as concept art or sound but I would not suggest it for programming as subversion works much better ( this is mostly because dropbox will instantly update anytime there is a change including very minor changes so every time you run your program it will update all the files that changed which is annoying to everyone else sharing the same folder ). Students who work at multiple places, school and home, can use dropbox to store your work using the public folder and uploading files from the dropbox website.
If anyone needs help getting dropbox setup or would like some more suggestions on how to use dropbox post a comment here and I will get back to you.