Red Hat contributes to a broad spectrum of Linux packages, drivers, and of course the kernel itself [1].

One example is virtualization: the virtio stack is maintained by Red Hat (afaik). This is a huge driver behind the “democratization” of virtualization in general, allowing users and small companies to access performant virt without selling a kidney to VMware.

Also, Red Hat contributes to or maintains all of the components involved in OpenShift and OpenStack (one of which is virtio!).

[1] https://lwn.net/Articles/915435/