Dropbox has nailed a use case that I've struggled with for a long time. I have a computer at work. I have a computer at home. Generally I keep files in sync via version control (Subversion, Mercurial, etc) but this is cumbersome for large files, specifically mp3 files. I consume a lot of music, digital and otherwise. How do I keep my music in sync between computers?
Simple, I use a Dropbox directory on each machine. This directory seamlessly acts like any other folder in the Finder on Mac OS X except that it is magical. Here's how it works. I have two private Dropbox directories for music. One is called from-work and the other is called from-home. As you might guess, I copy over any mp3s that I buy or download while at work into the from-work folder and visa versa. Due to the time bending wormhole like properties of Dropbox, files within those folders are instantaneously synchronized on each computer. In fact, the transfer is faster than most other things I do on the Internet, probably because it operates via the efficiently distributed Amazon S3 service (I believe). Each time I hop on my home computer I just move the new mp3 files in the from-work directory into my iTunes library and happily continue enjoying beautiful music. I'm sure I will get schooled in the comments but I have yet to find such a dead simple and efficient way to keep my music in sync.
If you've noticed the referral part of the URL I've linked to you might be wondering about this little pyramid scheme. Yes, Dropbox has lured me in to this scheme by offering me more free space if you like what I'm saying and start using Dropbox yourself from now on. I wouldn't be such a sucker for this if I didn't honestly find this service to be revolutionary to my daily life. So why don't I just pay for more space? Well, it's actually kind of expensive and since I use Dropbox in a transitive manner I rarely hit the space limitation. I know Dropbox is not really a backup service but if they priced themselves more competitively against Mozy or Backblaze then I would be more inclined to pay for more space. I would love to use Dropbox for backup but it is not currently cost effective for that.