After the client app is installed, you don't need to use a browser interface to use DropBox, just the DropBox folder in your file manager. The app works well with Windows, OS X, and Linux using the GNOME interface (KDE users can still use DropBox, but it's buggy; the browser interface is recommended).
All users of DropBox get 2Gb free right off the bat, plus another 250Mb if they complete the sign-up process. From there, things get pricey. $99/year will get you 50Gb capacity, and $199/year 100Gb. Again, a little better than Apple and Ubuntu, but far more expensive than Google.
The deep integration is very useful, but likely not enough to justify the price tag for big storage needs.
If you are looking at storage from purely a fiscal vantage point, the clear winner in the free storage category right now is Windows Live SkyDrive. 25Gb of free storage far exceeds any other offering. The only problem is that the storage does not scale. You can not get more online storage from them even if you were willing to pay for it.
In terms of scaling for cost, Google Docs offers the best value for the buck. Its pricing structure is superior to any of the other offerings here, as is the scalability. Unfortunately, the storage is mired in the kludgy Google Docs interface that makes file management almost painful.
If you are not looking for a lot of storage, then Ubuntu One and DropBox are superior interfaces for their respective operating systems and well worth looking into.
As storage becomes cheaper and the market more crowded with storage vendors, expect all of these offerings to either come down in price or go up in capacity. Like real clouds, this is one area that will be constantly changing.