The free dropbox plan comes with 2GB of data. If you invite someone new to dropbox, both they and you get an extra 500MB, up to 16GB for free.
If you’re a first time dropbox user…
Use this link to get an extra 500MB. (I get an extra 500MB too!)
If you are an existing dropbox user:
Open dropbox.com in your browser and login.
Click on the icon at the top right of your dropbox and select ‘Settings’
Select ‘Plain’ then scroll down to ‘Invite a friend’
Each friend who joins will add 500MB to your account!