OS X Lion: One of the problems with iCloud is that it only works if you're using the Apple programs it supports, but Macworld forum poster CHM figured out a way to sync and backup your documents across computers without using iWork.The first thing you'll need to do is enable the Documents and Data section in iCloud under System Preferences. Even if you've already done this, you'll also need to load up an iWork document and save it, as this will create a Mobile Documents folder in your Library. It's a hidden folder, so to navigate there, you'll need to hit Go > Go to Folder in Finder, then type ~/Library/Mobile Documents. This is your sync folder and anything you drop in here will sync across Macs with Lion installed. To make things easier on yourself in the future, you might want to drag the Mobile Documents folder to the sidebar or dock for easy file-dropping. The only real catch is that you won't be able to access these files on iOS or via the web interface.
It's not as simple to use as Dropbox, but it is an easy way to utilise free space you already have.