http://hive.ccs.neu.edu/
In a nutshell, here's how it works, the Hidden Volume Encryption (HiVE) is a kernel module that starts by encrypting the entire drive, but then within that encrypted drive, there is another encrypted volume hidden within the free space using a different encryption key. You can have as many hidden volumes you desire, and as many keys as you desire.
Simple, elegant, amazing.
Currently tested within kernel 3.13.6, and kernels must support drive mapping.