Depends on what you need in the mobo.

On my rig I'm using the ASRock 4CoreDual-Sata2 R2.0 for the motherboard
and Intel's Q6600 as the processor. For lower versions of Ubuntu (I
think version 6 and below) the onboard audio is not supported, but every
thing else works fine on it. It's a good motherboard too if you want to
use outdated equipment but still upgrade to newer stuff (for example,
it's got both AGP and PCIe ports, can support four IDE or SATA drives
(or 8 if it's all together, I think), as well as either DDR or DDR2 RAM
(though it warns you not to mix them)). It's not a high performance
board (your memory cap is 2gb of DDR2 667, and the FSB isn't that
great), but it's a great bargain board if you're trying to build a
system to last for awhile.