AMD has has been very actively supporting coreboot for a few years now. They have been donating much of the source to support AMD cpu's and chipsets.
AMD SimNow simulator is a fast and configurable x86 and AMD64 dynamically-translating instruction-level platform simulator. With SimNow users can connect complex software models to form a PC platform emulation environment. SimNow™ emulates AMD Athlon™ 64 and AMD Opteron™ uniprocessor and multiprocessor based systems that run several commercial operating systems and applications.
AMD Geode Porting Guide is a collection on information to help you on your way to porting coreboot to an AMD Geode platform.
AMD frequently has active coreboot developers in the #coreboot channel on irc.freenode.net.
VIA is helping the coreboot community support many new Epia's, mainboards and chipsets with several code donations and open programming guides. With key input from VIA, coreboot will soon support over 30 more Mini-ITX mainboards. These mainboards are made by VIA, Jetway, Phitronics and MSI featuring the VIA C7 cpu, CN700 and VT8237 chipset.
Linux VIA Portal aims to expand cooperation with open source communities by providing drivers, chipset programming manuals and source code for select Linux distributions to technical software developers.
VIA has recently donated v2 Cache-as-Ram (CAR) source for the C7 cpu.
VIA posted programming guides for CX700M/VX700 and VX800 at the VIA Download Portal
VIA help fix raminit for the CN700 so coreboot can now support all the C7 + CN700 + VT8237R boards (about 30- 40 currently in production) in V2 and move them into V3.