La-Mulana
Lyle in Cube Sector
1213
Those 3 (in that order) seem like a good choice for the GP2X. Don't know how they are implemented, what the hardware requirements are or if there's source available.
Many from the list (like Clonk, Cave Story, Ur-Quan Masters, Beneath a Steel Sky, Nethack) are already available for the GP2X.