[youtube=”http://www.youtube.com/watch?v=oU2PkF9QLTQ&feature=youtube_gdata”]
Direct from MAKE Volume 16, this project shows you how to mod your printer so that it prints with invisible ink.

More information, including a PDF of the instructions can be found here: http://blog.makezine.com/archive/2011/04/how-to-for-secret-agents-invisible-ink-printer-mod.html