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.
More:

RSS for MAKE
You can also do the following simple procedure:
Wet a sheet of paper – Put a dry sheet on top and write message with a ball point pen or pencil.
Destroy top sheet, (to keep the secret), when dry the lower paper seems blank so write something innocent on it.
To develop the writing wet the sheet again and the secret message is visible when you hold up to the light.
Good one! Thanks. I’m thinking some more secret ink content may be in order.
Follow MAKE
From the Maker Shed
Launch Price: $9.99
Make: School’s Out! Summer Fun Guide
This Summer Fun Guide is brimming with over 50 projects for kids of all ages. You’ll also find inspiring young maker profiles, as well as tool reviews written for and by kids! Plus, it’s our 3D issue, complete with 3D glasses and tons of super-cool 3D photos!
Read More →