Getting Started - 250MB
You must complete at least 5 out of 7 getting started tutorial steps.
Referrals - 500MB or 1GB Each
You can receive credit for up to 32 referrals.
Social Tasks - 640MB
Complete some social media tasks and you will receive 128MB for each one.
Camera Upload - 3GB
You get 500MB for your first photo and 500MB for every 500MB of photos/videos that are uploaded using the Camera Upload feature.
Dropquest - 1GB
Dropbox puts on a fun puzzle adventure called Dropquest each year. You will earn 1GB if you complete it and the top finishers can earn even more prizes.
Guides for Dropquest 2012 can be found here.