Infrared light is largely invisible to the human eye but can easily be seen by most cameras with slight modification. Digital camera sensors have great sensitivity to IR but typically contain a filter which blocks out most infrared light (for improved picture color).
This page details how to remove this filter and build an IR illuminator, an array of IR LEDs that acts like an invisible (or nearly invisible) spotlight. This is wonderful for exploring the dark, streaming at night and filming those midnight police raids.
Even if you already have a camera or lens sensitive to IR a homebrew IR illuminator is a very cheap and easy alternative to commercial options.
 What You Need
IR LEDs, for a spotlight 10mm models like these work well Ebay, $5-20
(These LEDs are 850nm, they give off an extremely faint red light when humans look at them directly.)
Your old webcam or digital camera - free!
1. Connect LEDs in a way that works with your power source. I used 12 LEDs (6x2) with a 9V battery because it's what I had handy.
2. Take apart your cam, remove the IR filter from the lens. It should look like a square of tinted glass. This is really easy with the EyeToy, but depending upon your camera make and model it may require more substantial modification of your lens. Instructions.
3. (Optional) Should you desire, you can replace your camera's IR filter with a piece of unexposed, developed film negative to block visible light.
4. You now have an IR illuminator and IR sensitive camera, explore the dark!
 Action Shots
One small photo. If you have others please share!