I worked on Ubuntu One in the last year or so of its life - it’s great to see this, and it’ll be interesting to see if anyone forks and grows it.
For some context, this is the server side of the service. The clients were always fully open (or at least since well before I started). The last versions of the clients used PyQt and ran with a single UI code base on Linux, Windows, and Mac OS X.
Ubuntu One never took over the world, but it did have a lot of users and quite a bit of their data under management, and it performed very well. I have a lot of respect for the team that worked on this stuff, and what I saw of their code was solid and well-considered.