We already covered SpiderOak, but only on a quick test, because back then, I didn't see the need for another tool like Dropbox. Read on to see why Dropbox might actually not be enough!
In case you don't know, SpiderOak is an online backup, sync, sharing, access & storage solution for Windows, Mac OS X, and Linux (Ubuntu, Debian, Fedora, SUSE). They offer 2 GBs of space for free and you will be enjoying all of the features SpiderOak has to offer. By referring 3 friends, you can get 3 more GB of free space (1GB per person referred).