Truecrypt is a very useful free tool available for both windows and Linux platforms , using truecrypt you can create encrypted volume and folders. Even you can fully encrypt your hard-disk, this is also very useful as you can encrypt your USB mass storage deices e.g. Pen drives or external hard-disk , so in case of lost of your drive data will be safe.
See the previous post regarding Truecrypt , I have recently found an Android App that I am sharing here, this is called EDS lite using this Android Apps these encrypted volumes or folders inside your USB disks can also be read in your android devices like Mobile or Tabs. So now you can travel with all data stored safely inside your USB disk using truecrypt and same can be viewed inside Laptop with truecrypt installed or within an Android devices that is having EDS lite Android Apps installed.