You could put your documents in a TrueCrypt container. Its namesake has been retired for unexplained reasons, but since the project was open source, an updated version is available under the name of VeraCrypt.
If Dropbox compatibility isn't mandatory, you could ditch it altogether and instead prefer SpiderOak or iDrive. While designed primarily for backups, both offer a sync function as well, and their free account give as much space as Dropbox. I use both, both had serious issues at a time, now solved, so there's no clear winner.
But while TrueCrype was proven "not insecure" by Bruce Schneier a few yers ago, it isn't that great in the context of cloud syncing as you need to close the container before syncing. CloudMask requires that you verify your identity through… Google. Why ? I don't know, I haven't read their ToS. Too bad if you don't have a big G account.
Viivo states it "doesn't break the way cloud storage providers work" (whatever that means, but I doubt it in the case of Dropbox, since this provider does save on disk space thanks to deduplication, i.e. compare files you send to files from other users so as to keep only one copy - they have access to your data by design.), but their Mac client requires Mac OS X 10.7 Lion at a minimum. You may want to keep that in mind if you held onto Snow Leopard for different reasons (Compatibility with PowerPC applications, better support for CUPS-shared printers, more elegant GUI, nimbleness, you name it.)
As with any software you don't have control or oversight over and especially with security software originating from non-privacy-respecting countries (US, Canada, France, probably others as well), make an effort to read the ToS. If dealing with data that must be kept private as part of a business, the better if you can have an attorney take a look at them.
