The "Mossad Pen" from Shomer-Tec ($28) writes with what appears to regular, visible ink. Hit your scribblings with a hair dryer, however, and they'll disappear. Pop the blank pages in the freezer and they're back! It's exactly the sort of high-tech invisible ink that I'm sure the Israeli intelligence cadets are using to pass messages around in class right now.
Can you believe we've done this site for nearly two years and we still don't have a "Spy" category? I think that says a lot about what gadgetry has become over the last couple of decades.