This information is partially, but not entirely, self-serving.
Dropbox is awesome. A folder on your computer (which you can put anywhere) syncs with the online Dropbox servers (a free account gives you 2 GB of space). So files in your dropbox can be accessed from any computer with internet access through the web interface. This is great for keeping important files backed up off-site or just so you always have access to particular files or folders. What's more, inside your dropbox folder is a folder called Public. You can right-click on any files in that folder and click "copy public link". You can then paste the link from your clip board into an email, IM, message board post, facebook wall, etc., and anyone who clicks the link (whether they have Dropbox or not) will download the file.
The best part about Dropbox, though, is the sharing. Any folder in your Dropbox folder can be shared with anyone else in the world running dropbox. If I have a folder called Arrested Development in my Dropbox folder and I share that folder with you, an Arrested Development folder will automatically appear in your Dropbox folder and the folder on your computer will stay synchronized with the corresponding folder on my computer. Any files you put in (if I give you full read/write access) will automatically appear on my computer and vice versa. And it all happens behind the scenes. It's something where you just set it and forget it. Very cool.
So now for the (not entirely) self-serving part: If you want to sign up for Dropbox, don't just go to the website and sign up. Have someone officially recommend the service to you. That person will then get an extra 250 MB of free space added to the initial 2 GB. But here's the kicker -- so will you! If you go to the website right now and sign up for a free account you'll get 2 GB of space. If I send an invitation to your email and you click the link to sign up you will start out with 2.25 GB of space, right off the bat. And, of course, I'll also get an extra .25 GB added to my total.
So, anyway, awesome service. I've been using it for a couple years now and never had a complaint. If you're interested, sign up under someone you know who already has an account (remember, you can't just specify that person during the sign-up process--he has to send you an invitation first!). And if you don't know or prefer someone else with an account, I would be happy to be that person for you--just send me a PM with your email address.