What's the most creative SaaS marketing campaign you've seen?

SaaS companies are getting more creative with their marketing, and I'm always looking for inspiration. One campaign I loved, largely because of its simplicity, was Dropbox's Refer a Friend campaign. They offered users 500MB of bonus space for each friend who joined and installed Dropbox, and the friend who referred them got the same. The simple yet brilliant strategy resulted in 3900% growth in just 15 months, primarily driven by referrals. What's the most creative SaaS campaign you've come across? What made it stand out to you?