I can't say enough good things about it. Dropbox provides transparent, constant, internet-based backup and synchronization of a folder that remembers previous versions, with a few extras thrown in to allow easy sharing. It works on the Mac, on Windows, on the iPhone, the Android, the web, almost anywhere you can get to the internet. And it starts out free.
But what does this mean?
It means that you get a folder on your computer that acts like any other folder. You can move or copy files in and out just like any other folder. And because it is a local folder, it's just as fast as any other folder on your local hard drive. But this folder has the magical property of being backed up, to the internet, in the background, and that backup is updated every time you make a change.
And that backup is tied to a user account and login. Add another computer to the account, and anything you move into the dropbox on one computer ends up on the other computer. Or it's now visible via the dropbox app on your iPad or mobile phone.
Or in a pinch, at a friends house, you can log in via the web and download the files that way.
The fact that it does this transparently in the background is awesome enough, but that's not all! Every time you make a change, it keeps the previous versions for the last 30 days - so you can go back and recover that file you deleted or overwrote while on the road.
And that's still not all. You can share any folder in your dropbox with other dropbox users that you specify. Now, anything you move into that subfolder shows up on their computer.
But, you ask, what if they're not on dropbox? Or if I don't want to share a folder with them? Well, there's also a "public" folder. Move or copy a file in there, and right click on it, and select "Copy Public Link". Now send them the link, and they can download the file directly from Dropbox. No need to fiddle around with yousendit, etc. for those occasions something's too big to email.
Dropbox starts free at 2GB of storage, and you can pay for more. You can also refer your friends to get extra free storage for everyone who signs up a new account using your referral link.
And joking aside, it is an outstanding product that they constantly improve to make it faster, smaller, and more reliable than it already is. I evangelize it every day, and I keep all of my "current project" files within my dropbox folder. If I lose my laptop away from home, I'll be able to recover all my current stuff up to my last internet connection, and anything older is archived to other backups anyway.