We all know that you can get more space for your Dropbox account by referring others but there are a couple other ways you might not know about. First you can connect Dropbox to your Facebook & Twitter accounts, send Dropbox some feedback and Tweet about it for an additional 756 meg of space (And if you remove Dropbox from these services after adding it the additional space will stay). Visit http://dropbox.com/free to get the additional space.
If you have access to a .edu email address you can get 500 meg for each referral instead of the normal 250 meg, in addition this is retroactive so you will get additional space immediately for anyone you have already referred. The other benefit to this is that the amount of free space you can earn is raised from 8 gig to 16 gig. To sign up visit http://dropbox.com/edu.