Encrypt your Dropbox (or parts of it) on Linux
UPDATE: EncFS is considered to be insecure (see this Audit), i will use eCryptFS instead, see this manual. If you still want to use EncFS use gnome-encfs-manager instead of gnome-encfs below.
I have some sensetive Data on my Laptop i want to sync with other Computers i own, found this Howto on howto do that. Theres also a Windows "port" of encfs - safe, didn't test it tough.