Dropbox is awesome. I have a 50GB account and use it for everything. I keep an alias on my desktop and host all of my current working files there.
If something happens and I accidentally save over a file, you can use the “Restore previous version” feature – which is the bomb. http://www.dropbox.com/help/11
Sugarsync is another service like dropbox, but has an interface that is a bit more complicated. The good part about sugarsync is their free account is 5GB. Also, you don’t have to put files in a specifc directory like you do for dropbox. I use Sugarsync to back up my Mail OS X Folder (username/Library/Mail and username/Library/Mail Downloads) and also my fonts (username/FontExplorer). Those are two directories that can’t necessarily be moved to the dropbox directory. I usually only leave sugarsync on overnight. Having Dropbox (I turn it off sometimes if I’m working on large files and don’t want them to upload immediately).and Sugarsync on all of the time might decrease your CPU speed. If you have a fast internet connection though, no worries!