Only open for business: Linux

Linux hardware support written/maintained under NDA
UltraSPARC III support
Cassini Ethernet
Marvell USB wireless
Several webcams
Syskonnect/Marvell Yukon II Ethernet
ALI south bridges
Serverworks/Broadcom south bridges
SiS south bridges
ULI south bridges
Intel PRO/100 Ethernet
NVIDIA SATA
and more besides...

Linux people have caved and made getting hardware documentation for many things near impossible