I'm a semi professional retro hardware / software modder who mostly does this as a hobby that turned into a small side business.
My street fighter 2 arcade hack running on an authentic capcom sf2 arcade board:
http://www.youtube.com/watch?v=CjKWb_cYzKY
My "for the lulz" alternate street fighter 2 arcade hack running on a capcom sf2 arcade board:
http://www.youtube.com/watch?v=si5Xn97LtgM
Castlevania 3 Japanese cart running on my av famicom with true s-video and a stereo sound circuit that includes proper mixing and bass / treble adjustment. I've since upgraded the video quality of this system but the difference would be hard to notice on a youtube video.
http://www.youtube.com/watch?v=BN0lQf02dDY
Starfox 1 running on a gsu 2 superfx chip overclocked to 2x speed. This video also shows the difference when I switch the clock speed real time between 2x speed and default clock speed the framerate goes from 2x to default and vice versa.
http://www.youtube.com/watch?v=brdYj5iGmL0
A full playthrough of starfox 1 on the gsu 2 cart running at 2x speed to demonstrate that the game is perfectly playable at this speed:
http://www.youtube.com/watch?v=PRwtrdPkvJA
A recording comparing the audio from my va1 model 2 sega genesis before and after I replaced the audio circuit with the amazing tiido crystal clear audio circuit.
http://www.youtube.com/watch?v=MMes1N-M2z0
Old videos comparing a regular nes to one of my modified models. The video quality has since been even further improved.
http://www.youtube.com/watch?v=tIJ2gRC77qA
http://www.youtube.com/watch?v=7v7e-bjB13s
http://www.youtube.com/watch?v=RLGo_P-MNG4
My snes super scope that I modified to run off of a wall outlet instead of using batteries.
http://www.youtube.com/watch?v=hfdsx6Z35j4
An old snes region switch mod I did:
http://www.youtube.com/watch?v=75oBq4p5IXU
Things I should make a video of include showing my custom built arcade supergun in action as well as recording a video of my model 1 sega genesis where I rebuilt the entire video circuit / video encoder.