Dynamite Dux (Amiga): the music of the trainer is the reason I learned to play piano.
... the game itself, not so great.
SNES: getting to re-play games in 60hz/RGB/stereo after years of playing 50hz/RF/mono nastyness.
Shadow of the Beast II (Amiga): when you discover that strange warp/bug right before the dragon head who asks you for a password (you jump into a rock off a branch and fall through the screen into a glitched copy of the level). Strangely I've never read about this in any FAQ.
Street Fighter II (SNES): when play an opponent so good the fight becomes like a game of chess. Also, playing as the bosses in the various hacked versions, before SF2: Turbo came out.
Bomberman 2 (SNES): when play opponents so good the match becomes almost like a game of chess.
R-type (Amiga): when you get tons of powerups and your ship is basically pimped to hell with weapons.
Turrican II (Amiga): when you complete it and are rewarded with epic music (as you have been all the way through).
Megadrive: when you power on at it says "sayyy-gaaaa"
MAME: realising that that fleeting experience of standing in a dingy arcade of paying good money for a few minutes experience of a decent looking game is now something you can enjoy in the comfort of your own home for free.. epic.
Super Pasofami: when Mario World boots and you realise whoa - this "emulator" is not a hoax!!!
Mortal Kombat (SNES): the hacked "blood patch" version that gives you red blood instead of green.
Super Gun: booting up a JAMMA PCB and for the first time in your life seeing "INSERT COIN" on your TV!
Unboxing a Super Wild Card 2.8CC as a kid at xmas and then slowly overdosing on SNES games over the years right into adulthood.
Ridge Racer (PSX): the music. Epic. No game before or after had music like this. WHY.
Ridge Racer Hi Spec (PSX): running this demo and realising that the PSX is actually capable of double the framerate and double the resolution of the launch title that defined the system. Mind boggling.
Donkey Kong Country: those graphics.. on your SNES!
That moment you realise trainers exist for console games too, not just Amiga/ST.
Super Mario World (SNES): when you email -Pan- and he sends you a buggy unreleased trainer.
Tekken 3 (PSX): getting a phonecall at midnight to say do you want to come to London to get a copy of Tekken 3 from someone.. set off at 4am.. back at 6am. The days before downloading an ISO was feasible..
Super Mario Kart (SNES): when you put your banana in the right place to hit
bloody Princess before the jump so she spins and misses a lap.
Starfox 2 (SNES): the fact that this game never made it to retail. Playing the ROM.
Dreamcast: realising you don't even need a modchip to boot CDrs..
Decapping. ASM hacking. Dumping SNES graphics from VRAM before emulators existed.
My god my list could go on forever...