This had briefly come up in chat, but we didn't discuss it much, alas. I was thinking about inventions and innovations that started out in the civilian sector, but ended up being picked up by the military.
One example that immediately comes to mind: The caterpillar track was used on Holt tractors during WWI for hauling heavy artillery, but it had been invented for agricultural use. That didn't stop several countries from adopting the design after they saw how well it managed uneven terrain and the modern tank was born.