[gst]

Depends on the online-service. E.g., I trust tarsnap (client-side encryption, not under an open-source license but you can compile it yourself) with very sensitive data. I also trust Wuala (also client-side encryption) with semi-sensitive data, although it somewhat worries me that Wuala's source is not publicly available for reviews. I don't trust Dropbox due to the lack of encryption - that's why I don't really use it, even though I currently have a free account with 20 GB available.

[icebraining]

I just mount encfs over the Dropbox folder, it's perfectly transparent. It does need Fuse, so no Windows support, but I don't really need it.