I haven't used either, but http://sparkleshare.org/ and http://owncloud.org/ seem to be the best open source options. I may give them a try soon - I'm getting more and more concerned by Dropbox.
I use TrueCrypt to create encrypted files containing the sensitive data I want on the cloud. TrueCrypt is free and quick to install, so I can pretty easily access the files from any computer if I need to.
While this (and other things mentioned like EncFS) is a solution, it integrates badly with user experience. I cannot access my data via web interface or smartphone app and I value this very much.
It is a bit complicated, but you _can_ create smartphone application and web interface for the data encrypted on a server.