The one that looks by far the simplest to use is Dropboks. First of all, it’s free; you can get going straight away; the interface couldn’t be more straightforward; and of course it’s all secure. It’s definitely worth a look if you’re sick of using your Gmail account as a surrogate back-up online storage facility.
UPDATE: 18/07/07: The only slight problem with using Dropboks on Firefox is that it doesn’t allow right-clicks; or, rather it does allow them, but the right-click menu on Dropboks is automatically obscured by Firefox’s own context menu. I’ve come across this on a few sites now (including Yahoo! Mail Beta), and so far I haven’t found a work-around. If anyone knows of one, please leave a comment!