Depends on how much space you need.
Dropbox is my personal favorite, and offers 2 gigs free online storage, or 50 gigs for $10/mo or $100/yr.
You can also just make a crap load of accounts, eliminating the 2 gig cap
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Windows Sky Drive offers 25 gigs free, but upload size is limited to 50 MB per file. And you have to have a live account.
I prefer either of those to most of the others, because you don't have to remember links to files you uploaded months ago; you get the use of a folder system to maintain your files, rather than just uploading everything into outerspace.
That said, one that I've just recently discover is Mediafire. As a downloader, it is quite nice in that it doesn't cap you at all; unlimited downloads, unlimited uploads, unlimited bandwidth. And no sign-up required. Plus the upload cap is 100mb per file, which is at least an improvement over Sky Drive.