When you refer people to sign up for Dropbox, the service would kick you an additional 250MB of storage. So, if you sign up for the free 2GB plan you could potentially increase your storage to 8GB.
Dropbox just doubled the referral limit to 500 MB, and you can now get up to 16GB free storage added to your free plan. If you’re a paying user, the referral limit is 1GB per referral up to a max of 32GB.
The best part is the referral bonuses are retroactive. My storage just got bumped from 2.8GB to 3.2GB.