Here's a useful list of chipsets (found on motherboards) that are documented. Since it's documented, it's easy to add support for them & the drivers are less buggy.
If you really care about Open Source, contact the company & ask for hardware documentation/datasheets. We don't want source code: we want freely available specs.
(Signing an NDA is not an option, we also want to share what we know to other groups out there: *BSD, Linux, Haiku, Minix3 and many other groups in pure open source spirit)
ICH7ICH8 mobileICH9 ICH10
CMD/Silicon ImageCMD 648CMD 649SiI 680
Promise:PDC20579PDC40775 ATI:IXP700/800 SATA
Intel (complex and buggy)
Realtek (older chipsets like 8187)
(Still a work-in-progress)