The newest motherboard supported by Coreboot is the Gigabyte GA-945GCM-S2L. This micro-ATX i945 motherboard from the Core 2 Extreme / Core 2 Duo days has DDR2-667 support, Intel GMA 950 graphics, SATA 2.0, Gigabit LAN, and Intel HD Audio.
Support for this old motherboard landed in Coreboot Git last night. PCI/PCIe slots are working along with serial output but not working currently with this less than thousand lines of new Coreboot code is resuming from suspend. More details via that Git commit message.
The Gigabyte GA-945GCM-S2L is obviously still no longer produced but if you are wanting a cheap motherboard to play with Coreboot, this board can be found on eBay and second-hand web-sites for $30~35 USD.