A few years ago I stumbled upon this article
describing how to get free Dropbox space by using google adwords to make people click on your referral link to Dropbox. Each click and installation of Dropbox gives you free space up to a certain amount. My experience is that you can only advertise in USA and Canada since you are allowed to use the word “dropbox” in your ad and link. In Denmark you can no longer get coupons for free adwords credit, but you can pay a certain amount, 200 DKK, and get a coupon for 800 DKK which is only useable for a new adwords account (about a week old) that has not used a coupon before. But that should in my experience be enough to get two dropbox accounts fully maxed out on referrals.
My settings can be downloaded and imported using the official adwords editor to get you started:
Play around the price per click values and max daily spending limit. Lower per click values means cheaper prices, but longer time before your ad gets shown, if at all.
If you use my settings, remember to change the actual referral link with your own, else you will not get any referrals.