Console Games I've completed (Or at least what I can remember at the moment... I might edit this post later to add more when I recall them)
On Atari 7800 (Most games on this system type are unbeatable, since they continue on until you run out of lives)
Only Galaga (Complete Wave 99, only to return to Wave 1) for now.
On NES
Monster Party
Blaster Master
Shadowgate
Contra
Paperboy
Super Off-Road (Completed all 99 races without continuing)
Faxanadu
Super Mario Bros. 1
The Legend of Zelda 1 & 2 (Adventure of Link)
Double Dragon 1 & 2 (The Revenge)
Swords & Serpents
On SNES
Eye of the Beholder
Super Mario All-Stars (All 4 games, & even World 9 on The Lost Levels)
The Twisted Tales of Spike McFang
Dungeon Master
Chrono Trigger
Starfox (Beating ALL 3 courses took awhile, but I did it!)
Super Mario World 1
Romance of the Three Kingdoms III: Dragon of Destiny
Final Fight 1
Vegas Stakes
Super Mario Kart
Uncharted Waters: New Horizons (Only Otto's storyline for now)
Ultima VI: The False Prophet
Breath of Fire 1 & 2
Secret of Mana
The Legend of Zelda: A Link to the Past
Super Mario RPG: Legend of the Seven Stars
F-Zero (Only Knight League on Master difficulty as of now)
Stunt Race FX
Donkey Kong Country 1 & 2
Super Castlevania IV
Super Metroid (Fastest completion time is 1 Hour, 48 Minutes)
Mega Man X 1
And, if we're counting emulations...
Umihara Kawase
Shin Nekketsu Kouha Kunio Tachi No Bannka
As for Nintendo 64...
Only Starfox 64 (I only have a few N64 games for the time being...)
On Playstation (PSX)
Legend of Legaia
Threads of Fate
Grandia
The Legend of Dragoon
Azure Dreams
Megaman Legends 1
Metal Slug X
Crash Team Racing
Team Buddies (Funny how the team colors here are EXACTLY like Halo 2's set)
Spyro The Dragon 1, 2, & 3 (Ripto's Rage & Year of the Dragon)
Legend of Mana
Frogger PSX & 2 (Swampy's Revenge)
Roll Away (A.K.A. Kula World/Quest)
Myst 1
And, if emulations count, Umihara Kawase Shun
On Xbox
Star Wars: Knights of the Old Republic 1 & 2 (The Sith Lords)
Blinx: The Time Sweeper 1 & 2 (Masters of Time & Space)
Halo 1 & 2 (Both on Legendary, though for the 1st game it was in Co-op Play)
Splinter Cell 1 & 2 (Pandora Tomorrow)
Dungeons & Dragons: Heroes
Brute Force
Destroy All Humans 1
Ghost Recon 1 (& Island Thunder)
Metal Slug 3, 4, & 5
Oddworld: Munch's Oddysee
Spongbob Squarepants: Battle for Bikini Bottom & The Spongebob Squarepants Movie
Sega GT 2002
Jet Set Radio Future
Xbox "Emulations"
Mega Man Aniversary Collection (MM1 through 8, plus The Power Fighters & The Power Battles)
Sonic Mega Collection Plus (Sonic 1 through Sonic 3D Blast, excluding the GG titles, plus Ristar)
On Gamecube
Mario Kart: Double Dash
And, in "Emulations", TLoZ 1 & 2 (Again)