Today I started playing around with BitTorrent Sync. It may very well be the best piece of software released this year.
It allows you to do flexible synchronization between any number of machines using the BitTorrent protocol. Setup is ridiculously easy. You share directories with yourself (on other PCs) or with others by simply pasting in a secret code (which is 40 characters) on the other end. There is support for Windows, Mac, Linux and FreeBSD.
Since all the storage is on your machine, there are no limits. You don't have to set up an account anywhere and your data is never stored on the cloud so there's no security risk there. Data is encrypted in transit.
It's a great way to share data among friends, or to maintain a constant offsite backup of your important data. You could sync your "my documents" folder between your PC and your spouse's, so you'd have a backup in case of a crash. You could sync your family photos to your mom's PC, or to work, and you'd always have a backup and mom could have your photos minutes after you pull them off the camera, with neither of you having to do anything.
I think it's pretty brilliant.