Earlier quoted context omitted.
If you're concerned about your personal data, and not so much about targeted attacks against you, I'd say stay with Dropbox and just use EncFS. You can do this on Linux (and presumably OS X, as well) fairly easily. On Windows, there is a single-developer port of EncFS, which from what I've heard works fairly well: http://members.ferrara.linux.it/freddy77/encfs.html A quick search turns up a guide which (at first glan…
Playing the devils advocate here. What would stop the dropbox service from being able to collect the private keys on the user's computer?
I'd say the real world risk of this is (fairly) low, however if you're legitimately worried about it, you can use EncFS over a network share or FUSE file system, to your own systems. In which case you'd be using entirely open source software.
If you're that worried about security though, you'd probably be better off using a container based encryption method, anyway, as it wouldn't leak timestamps, file sizes and other data which could be sensitive. EncFS has some known issues with metadata leaking, but it's a decent solution for most general use cases.