As far as crime-fighting costumes go, the Watchmen's Rorschach has one of the most interesting. As his name implies, his mask is an ever-shifting inkblot that changed from panel to panel in the comics, and by use of special effects in the movie. Turns out you could recreate this effect at home. All you need is some special paint.
Thermochromic paint is temperature sensitive, so when applied to a cloth mask it would react to, say, someone breathing through it. That's how YouTube user guinness0507 achieved the same effect as all that Hollywood movie magic, and for paint that supposedly cost him around $70 (which would be enough to make 50 masks, apparently). You've seen thermochromism at work before — ever played with a mood ring as a kid? Same concept.
The end result is creepy and effective:
Here's a video of guinness0507 explaining the process: