Dropbox is really awesome. I'm very impressed with how well they execute. Lots of companies big and small have offered the same basic service, but nobody has built a product nearly as good.
What about Live Mesh? (When assumed that all your computers run windows). As I see it, there is basically all the functionality of Dropbox, plus 5GB free space, ability to sync several folders, and remote desktop functionality.
I believe Live Mesh has an OS X client now. I tried it (on both OS X and Windows), and while it has most of same basic functionality as Dropbox, I found the interface off-putting.