TrueCrypt works. The client program needs polish (and I wish it could resize on the fly), but it once it mounts an encrypted volume, that's that. Even Dropbox's incremental sync works with TrueCrypt.
I don't mind my recipes for oatmeal cookies falling into the wrong hands. But the new junior food scientist at Coca-Cola who decided to store a copy of the secret recipe on their Dropbox folder so they can do some testing at home might be forewarned of this problem. Or for a real world example, the poster a few comments below who stores student grades on a Dropbox folder. I cringe.