The Dropbox Blog:

For every friend you invite that installs Dropbox, you’ll both get 500 MB of free space. If you’ve got a free account, you can invite up to 32 people for a whopping total of 16 GB of extra space. Pro accounts now earn 1 GB per referral, for a total of 32 GB of extra space. Have you already invited a bunch of people? Don’t worry. Within a few days, you’ll get full credit for every referral that’s already been completed. Boom!

Nice little bonus for those of us who use Dropbox.