When looking out at the Linux and greater Open Source world, examples of “coopetition” stretch out to the horizon. Ceph, Crowbar, Qt, GNOME and, heck, even the Linux kernel itself are developed by individuals and companies (who would, often, be in competition) working together.
Live patching of the Linux kernel (being able to patch a running Linux kernel without needing to restart) being another great example. In April of 2014, the engineers at SUSE submitted the code for their solution (“kGraft”) to the kernel.