Wuala: http://www.wuala.com

Data is encrypted with your password on the client side, your password never leaves your PC. They published a paper on their security implemenation: http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=4032...

It allows to synchronize multiple folders and it gives access to a certain number of previous versions. You can also share folders with friends, publicly or via a secret link.

It's cross-platform: Win, Mac, Linux.

I'm a happy customer since more than a year (it's free up to 2GB though) and I wonder why so few people know about it.