I'm an OpenBSD developer in my spare time. For the most part driver related things in the kernel.
Documentation is a vital element needed to write decent drivers.
Here is a list of some of the things companies don't want the world to know about for whatever reason that have hindered the efforts for people working on free operating systems.
SiS southbridges SiS 190/191 Ethernet ULI southbridges NVIDIA southbridges NVIDIA graphics NVIDIA Ethernet Adaptec RAID controllers 3Ware RAID controllers VIA 802.11 wireless Intel 802.11 wireless Intel BIOS Writer's Guides TI 802.11 wireless Atheros wireless AMD 802.11 wireless AMD FCH southbridges Conexant 802.11 wireless Broadcomm 802.11 wireless Marvell Ethernet Marvell 802.11 wireless GCT Semiconductor 802.11 RF transceivers Silicon Laboratories 802.11 RF transceivers