Can’t for the life of me work out how or why this thread idea occured to me at this time and with this much drink in me. Anyway…
Memorable computer games I’ve never completed are…
‘Fairlight’ on the amstrad cpc 464.
‘Prince of Persia’ (first played on a portable red-and-black screen computer the name of which escapes me)
Black and White (PC)
All the GTA games (PC and PSP)
Er, so what are yours? (memorable games you never completed)
Saddly, all computer games above Mine Sweeper remain unconquered by me.
Hitch-hikers guide to the galaxy. :wally I did get the bable fish tho 
I just finished a few long-unfinished games in the last year (Planescape: Torment among them), but I have a few to list:
I’m sure I have more but those are the notable ones that spring to mind.
Still haven’t beat Planescape yet, although I once got almost to the very end.
Same with Ico.
Dark Seed – a pixel-hunt adventure with art by H.R. Geiger. Repeated deaths with no intuitive cause. Very frustrating game.
The Last Express. I forget where I got stuck on that one. Neat game though. Someday I’m going to have to reinstall it on an old computer and give it another shot.
Unreal. Never finished the single-player game – Got bored with it and just played Botmatches if there was no-one around to play with. 
“Mercenaries” - I love the hell out of that game, and worked my way all the way to the end twice (lost my saved game), but the last mission is frustrating beyond imagination, and every attempt at it takes me at least a half-hour.
I don’t anyone has completed Black and White. The levels in that game were a mess, especially the one where they take your creature away. That was the end of single-player for me.
I haven’t completed the original Half-Life. Didn’t even try. I bought it TFC.
Pretty much the same here. I own hundreds of games but have only managed to complete a small percentage of the ones that can actually be completed.
The game I most recently got 100% on was Midnight Club 3: DUB Edition. The autocross events were a pain until I found vehicles suitable for my driving style.
BC’s Quest for Tires (for the Colecovision). Never did beat level four. Came agonizing close, once.
“the fools quest” for the pc… stupid puzzles…
My college roommate had Dark Seed, and it was fun to look at but annoyingly difficult to finish.
Aside from a mildy tricky reflex game near the end, The Last Express had only one big puzzle :
Defusing the bomb. The thing I hated most about it was that you had to burn the insulation off the wires in the bomb by lighting them with a match - a stupid stupid misstep in a truly great game.
Among the many nice things about that game is that you never had to go back really far if you missed something, due to the way the save system worked.
As for games I didn’t finish :
Super Mario Brothers. Never had access to a system long enough to complete it (and little desire to do so anyway)
X-Com (aka UFO Defence or something like that) - I only recently played this for the first time. I was having a great time, pretty sure I was losing but having fun barely hanging on, when one of my bases was attacked. I had no idea if you built it be difficult to attack (only one path to the surface), the aliens can’t actually attack you at all. The only thing to do is give up the base without anyone firing a shot. This pissed me off so much I gave up on it.
Rainbow Six : Rogue Spear - Played all the way to the last mission, which involved making your way through a long factory until you reached the last guy, who you maybe could shoot at once if you were very lucky. If anyone at all saw you, you lost a few seconds later. Since you can’t save in the middle, you have to go through the long boring part every time, and it comes down mostly to luck (since there are many long corridors you can’t hide in). After many game-hours wasting my time trying, I simply gave up.
Half-life : A bug in the game made it impossible to continue from a point about 3/8 of the way through. I’d had enough fun but wasn’t going to do it all over just to finish it off.
Deadly Rooms of Death : Once it got to the point that you had calculate moves more than 10 in advance, it got boring. I hate games that appear to be most easily solved by a computer.
Pengo
Lessee. I never finished FFIX, I got stomped by one of the minor bosses near the end, couldn’t go back and level up. Really wasn’t that interested in that FF storyline so I said the hell with it.
Half Life. Just couldn’t finish the level with all the platforms.
Driver. The last level was so unfair and I tried it so many times, I got to within a hair of hitting the spot and overdrove it.
BloodRayne. Final boss. Couldn’t put a dent in him. Looked up the FAQ, tried what they said. Didn’t work.
I had the same maze problem in Baldur’s Gate that Ferret herder.
Couldn’t live long enough to kill the final boss in Resident Evil 1. Got so pissed at the idiocy of RE:Nemesis and RE:CNV that I stopped playing them. Bosses you have to fight multiple times when ammo is limited do not make for fun games.
I almost gave up on Silent Hill 4: The Room. It was barely scarey, and mostly just annoyed me to no end. Finally I set the game to easy and played it through with the online FAQ. Ho-Hum.
There are a lot of games I gave up on becuase they deviated badly from their genre…A shooter becomes a stealth game, and actioneer becomes a platformer, etc. Sometimes this works to freshen the game, most of the time it is just incredibly annoying.
Pretty much every computer game I’ve ever played.
Funny Story.
Back in the day, back when we had our 8088 we had Kings Quest IV. My mom and I tried for about six months to finish the damn game and got pretty much nowhere. Off the top of my head, I think you needed 220 points (or thereabouts) to finish the game and we never got above 30. Friends of the family came by that christmas, saw the game, and asked to borrow it. They had the game for all of a weekend and finished it.
I’m one of those guys who dies on the first guy on Super Mario Brothers. (I’m surprisingly good at Tiger Woods '06 for some reason…)
The games I played a lot that I didn’t finish are:
-Age of Empires
-Age of Mythology
-Heroes of Might and Magic 1, 2, 3, and 4
-Warcraft (that’s the game with the weird green men right?)
-Both Command and Conquers
-Doom and Doom 2
-Myst
-Captain Comic
-Obviously Kings Quest IV
-There’s a few more first person shooting games I started but couldn’t get anywhere at…
haha, I’m hard-pressed to remember a computer game I have completed.
most notably, though, Baldur’s Gate, Planescape: Torment, Arcanum, & Fallout 2. Fallout 2, I got to the VERY last part, and I was too weak to beat the guard-people. I didn’t have the patience to go out and get the million jillion more xp I would’ve needed to level up so I said fuck it.
one that drives me crazy to this very day is Quest for Glory 4. fucking bugs EVERYWHERE.
My hat goes off to anyone who’s beaten the final bosses of Metroid Prime, and Metroid Prime II: Echoes.
I loved those games, but the final bosses are just insanely difficult.
Zork.
Ocarina of Time.
Any Final Fantasy game (I’ve never got more than halfway though, in fact. They’re good games, just too freaking long.)
Myst.