Invisible Light (Infrared Flash)
With inspiration from Weegee, I've made myself an invisible flash from a #87 Lee filter, a flash diffuser, and some cardboard and black tape. If you look right at it when it goes off you see a dim red blip, which isn't bright enough to even make you blink.
There's more information on making, testing, and using an 'invisible' infrared flash in the following sets:
• Making the filter
• Testing exposure
Testing focus [use a narrow aperture]
These tests were done using Kodak high-speed infrared film, which Kodak has announced will be discontinued as of December 2007.
Update 2009: I have an IR-converted D100 now, and the same results seem to apply; about a 1-stop loss in flash output. I'll get around to doing some more rigorous tests soonish!