Whitenoir converts any image to a strictly two colour monochrome image.
Using a really simple user interface it's possible to convert any image in just three mouse clicks.
The transformation is totally configurable. You can pick which two colours the resulting image should use and tweak the brightness threshold for the dark and light pixels to make your image more or less dark.
You can use Whitenior for creating sharp bitmaps from pencil drawings. No matter how faint the drawing is Whitenoir should be able to pick out the contrast and produce a sharp crisp bitmap.
It's very handy for creating stencil images. Just take your favourite photo of a boat or cat and you'll have a startling render in two colours in no time or why not try creating a lovely stencil of Keanu Reeves.
For best results it's best to start with a bmp, png or any other lossless type of image compression. If all you've got is a jpg then Whitenoir will still use it but the result may be on the fuzzy side.
I have no doubt this can be acheived in Photoshop, GIMP et al, but I'm just too lazy to find out how.