Ever since Google announced that you could upload arbitrary files to google docs there have been a bunch of shakeups in this market.  The profits of companies like https://www.dropbox.com are obviously threatened by this announcement.

Sadly, The big G didn't announce that it was providing a rich desktop app for drag-and-drop uploads, synch, etc.

A company called Memeo did.  I did some evaluation today, and I really love this app.

Their pricing is really attractive (just $9 /head per year).  However, I think their only hope of long term survival is a Google buy-out and somehow, I just don't see that happening as they Google will likely do something fuse based and/or build it in HTML 5 and just wait for the masses to catch up / drive the browser market.  Another open source solution I looked at but couldn't get working in 30seconds on my mac is http://code.google.com/p/google-docs-fs/.  It's a FUSE based implementation.  Likely not as feature rich, but nicely integrated.