As an Ubuntu user I rarely speak of the proprietary software I use. Not because it's a secret or because I find it embarrassing, I just don't want to start yet another discussion on the topic of open-source vs. proprietary software.
I'm using the fantastic DropBox, which is proprietary software - but still free. If you've never heard about DropBox check out this screencast at: http://www.getdropbox.com/screencast#screencast
After using Dropbox for almost a year, I must say it's great. Everything is very userfriendly (which sometimes can be hard to find on Linux) and the web interface for Dropbox is great. I highly recommend giving it a try.
I've heard that ifolder could do something similar, but I haven't yet had the time to check that out.
Steven Harms recently wrote a blogpost about DropBox
