There is no such creature. All depends on the unlockability of your CPU. That said, motherboards with the SB710/SB750/SB850 chipsets for the AM2+/AM3 platform would be your best bets.
As mentioned, you need a southbridge 710 or better with ACC (advanced clock calibration) to do it.
That being said, don't bet on a successful unlock. My board unlocked my last Athlon X3 and my current Phenom X2. However the 4th core of the X3 was unstable and made my rig run very slow. My X2 unlocked to an X3 but the 4th core was totally unstable so much that it wouldn't even boot into windows.