They make accelerated X display servers (for their own and other..) hardware on *nixs. They see their role in Linux as spreading the known stability of the Linux kernel to The DesktopTM.
They don't create open source or free software (natch, eh?) but from my experience their retail products are stable and support copious hardware.
Usually no better XFree86,though.
They produced the first hardware-accelerated 3D X server for Linux