Are you lucky enough to be a student, faculty member, or more with an .edu email address? If so, Dropbox would like to make your educational experience even easier. While Dropbox lets you refer people to the service for a gift of 256MB of free space per referral up to a maximum of 8GB of extra space (10GB total for a free account), those who verify that they have an .edu email address can refer people to the service for a gift of 512MB of free space per referral up to a maximum of 16GB of extra space (18GB total for a free account).
If you have already referred people in the past before you verify your .edu email address, the previous referrals will be upgraded to the 512MB size.
You can verify your .edu email address by going to this page, entering in your email address, and clicking on the verification link that will be sent to that address.