Here is a nice round-up of some simple sneaky inks. The old favorites like lemon juice and vinegar are there, but also some more sophisticated systems using stuff like cerium oxalate and iodine fuming.
From the pages of MAKE:
Mike Golembewksi shows us how to make an invisible inkjet printer in MAKE 16 – Spy Tech.
- How-To: Make an “invisible ink printer”
- How-To: Make invisible ink
- How-To: Analog text compression technique