I firmly recommend Tarsnap. Even though it's paid, it's fairly cheap (at least by European/US standards) since you only pay exactly what you use, the client application is open source and it's a one-man-show by Dr. Colin Percival, cperciva here on Hacker News: http://news.ycombinator.com/user?id=cperciva

It's not for everyone, though:

    At the present time, Tarsnap does not support Windows
    (except via Cygwin) and does not have a graphical user 
    interface.