For example you can use TrueCrypt and create/mount a drive volume that is fully encrypted while synced across a file share storage/sync service like DropBox.
The only downside is that you have to install the TrueCrypt application on your client device, which does limit is platform offering (currently, no mobile).
A better solution is to use .sparsebundles (Mac only) or something like BoxCryptor/encfs that encrypts files in the volume individually
I use encfs though, good suggestion.
BoxCryptor does look like a great alternative; however, it's not open source and does come w/ a price (literally and figuratively - file names exposed if using free version or drop $50 USD).
Have you used BoxCryptor? Curious what your experience has been. I'll give it a try.
The real problem with using a Truecrypt volume on Dropbox is concurrent changes to the same volume. It happens if you mount the Truecrypt volume on 2 machines at the same time. It will mess up badly.