Libreboot now supports the ASUS KCMA-D8. This isn't some new and exciting motherboard, but rather a motherboard for Opteron processors with the SR5670 + SP5100 chipset. On the bright side, at least this motherboard is still being sold as new from various Internet retailers.
The ASUS KCMA-D8 has dual C32 CPU sockets, six Serial ATA 3.0 ports, two PCI Express 2.0 x16 slots, eight DDR3 DIMM slots, etc. The motherboard is selling for around $280 over on Amazon.com.
The Libreboot support page indicates things are working but only text mode works, the IPMI iKVM module relies upon proprietary firmware, the SAS support requires a non-free option ROM, and there are some LRDIMM memory module issues.