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