True crypt has an interesting feature. You allocate encrypted 'drives' which occupy large files (the unused part is randomized). In addition to the primary encryption key, you can create a secondary key which encrypts data in the 'unused' portion of the file which is pretty much indistinguishable from the algorithmically randomized filler.

The primary key will unlock the 'active' data, there is no way for anyone to know that another key will reveal a second repository.

[It has been suggested that it's a good diversion to encrypt in the primary mild soft porn or somesuch, anyone searching your drive will assume that's what you are hiding.]