Dropbox Forums: The Official Dropquest Thread

And there's also the referral program:

    Each friend you refer to Dropbox yields 250MB of free space. 
    In total, you can get up to 8GB for Dropbox Basic accounts 
    (for a total of 10GB), and 16GB for Dropbox Pro accounts (in 
    addition to the amount you paid for already). All your friend 
    has to do is use the link you send to create a Dropbox account 
    through the Dropbox desktop application.
    
    Even better, if you're a student, we double the amount of 
    referral bonuses to 500MB per friend and up to 16GB of extra 
    space for Dropbox Basic accounts and 32GB for Dropbox Pro accounts.