The PRU is a fantastic bit of hardware. With two of these running at 200MHz, and direct register-mapped access to GPIO pins, there's a lot of cool stuff you can do with them.

For example, in a recent project, I used one of the PRUs to generate a precise 40MHz square wave clock signal with 40% duty cycle, and the other to read the signal pin of a camera module into shared RAM. It worked extremely well, allowing me to obtain camera data at hundreds of FPS, and freed up the main CPU to do some fairly heavy image processing - all without involving an expensive camera capture rig or an external PC.