I am currently using an ASRock ConRoe865PE and it works very good on linux. It is based on the good old Intel 865 chip using DDR1 ram and works nicely with my old AGP graphics card. Everything I tested so far works nicely. The only thing I did not test is onboard audio. The bios is not usable if you want to overlock, but for everyday usage it is perfect. Here is a lonk to the Product itself:
http://www.asrock.com/mb/overview.asp?Model=ConRoe865PE
The board works perfectly with the vanilla 2.6.18.6 Kernel I am using. All 2.6.18.* Kernel did work nicely, too.