Spider Oak: I wish there were some open source cloud client similar to True Crypt. Something that were really easy to use, working on every system and open source. Spider Oak's client seems to be still mainly a closed source product.
The whole thing currently sits on top of BitTorrent although I suspect I'll end up writing a forked implementation at some point to support some extensions I'm working on.
Active, dynamic syncing is great for some kind of data but not so important for others, and I feel like that's the hurdle that needs to be overcome. More interesting to me is painless, fast, robust, and configurable sharing of relatively static data. (Some versioning may be included but it won't be a first-class system.)
It is libre software and cross-platform. A number of front-ends address the issue of ease-of-use.
[0]: http://duplicity.nongnu.org/ [1]: http://duplicity.nongnu.org/features.html
cperciva posted why Tarsnap was more secure and cheaper than Duplicity a while back:
Of course it would still need a security review.