I thought I'd already found the best example of a BILLY hack already —these built-ins from Gwen of Makerista, which we posted early last year. Then this project came to my attention, and I've made room in my heart for two loves instead of just one.
Kristina of Ich Designer created a series of these sliding doors for her BILLY bookcases. They move on casters with the help of a u-shaped channel she affixed to the front of the shelves. IKEA bar pulls complete the job. She used OSB, which makes the DIY look modern and functional, while remaining super affordable.
Although the written instructions are in German (points to the reader who translates it for us), her photos are great and illustrate the process pretty well. Following along, you could probably recreate the project on your own.