Dropbox is a great service: it allows users to keep everything (yes, everything) in sync between multiple devices. I daily use Dropbox in my workflow to backup my Yojimbo library and share stuff with my friends and co-workers, and I'm very satisfied. If you're new to Dropbox, you can signup with this referrall link and, besides testing the service, you'll make me earn 250 extra MB. (and I will thank you in one way or another)
Anyway, there are many creative yet less known uses for Dropbox. In this roundup I've collected the best I found around the web, but if you know more please share them in the comments!