Since the NES emulator works pretty beautifully on my iPhone, I've been thinking about other emulators that might work as well -

Stella (Atari 2600) - the 2600 should require less power to emulate and, given its one-button+joystick controls, might be easier to play with. Heck, it could even use the accelerometer to cover the control scheme, you know?

Stella homepage-

MAME (Arcade machines) - older games might work out really well, like Galaga and Tempest.
MAME homepage:

I have no development expertise at all, but I'd be absolutely happy to help test anything anyone's doing.

Thank you to everyone who works on the iPhone, enriching the content available for all of us!