Old info for version 1.0:
So, here's a little something I've been working on the last couple of days: http://removed
These mappers allow players to save/load the game state in real time using Select+A/Select+B buttons. So far I've implemented NROM and UNROM (Castlevania, Mega Man, Duck Tales and so on) support.
The save state can also be saved to SAV files on the CF card. Note that the SAV file has to be 16KB for this to work. (Actually, for NROM games it should probably work with just 8KB, but I didn't test it.)
Currently saved:
- CPU 2KB memory + registers
- PPU palette + nametables + CHR-RAM + $2000 + $2001
- mapper state
"TODO":
- more mappers
- more complete save state (include APU etc)
- configurable buttons
- more save state slots (not really that useful because state can be saved to CF)
- hex editor
- the possibilities are endless

EDIT 2011-01-30: removed old link