Dropbox already offers you 2 GB of space for free upon signing up, but you can get up to 8 GB of additional storage if you take part in their referral program.
Each person who signs up via your referral link earns you 250 MB, so if you get 12 people to use your referral link, you'll get that extra 3GB.
However, there is a faster way. You can have an extra 250MB, right now, if you use my referral link (you can also use this alternative link) and sign up for an account.
This way, both you and I will get the additional 250MB – a perfect win-win situation.
Aug 28, 2009 update – eight days after putting up this post and a tweet, I got my first referral! The Dropbox team kindly dropped me an automated email (titled 'Dropbox referral status') to announce the fact.
Sep 03, 2009 update – My second Dropbox referral is in – I'm up another 250MB!
Nov 24, 2010 Update: Vlatte has written a post on how to get an additional 768 MB of free space by using your Twitter and Facebook account to promote Dropbox.