Hey guys, I realize the Wishlist is a great feature and all, but I hope people don't mind my talking up a few games that I think deserve some sort of recognition.
Rocket Jockey:
People barely even knew this game existed back when it came out, but it's one of the most original games I can remember playing. The gameplay is simple, you ride a rocket equipped with two extendable grappling hooks, competing in various arena matches, involving things like yanking your opponents to the ground and knocking them out, to a very alternative game of soccer, to simple racing. Insane learning curve, but one of the rare games that are actually worth the grief.
http://www.giantbomb.com/rocket-jockey/61-10145/
Silent Storm:
While perhaps not as old as a lot of other games here, it's still easily the best turnbased tactics game since Jagged Alliance 2. Set in the second world war, you command a squad under the authority of either the SOE or the Abwehr, and start out by carrying out pretty straight forward missions, most often behind enemy lines, soon uncovering a nefarious plot unfolding in the backdrop of the war. It's a strange game, but one that's well worth owning.
Also, very lame graphics restrictions means that you need a bunch of workarounds to play this with any sort of modern graphics card. I'm thinking the wizards of GOG could work their magic there somehow.
http://www.giantbomb.com/s2-silent-storm/61-10967/
Little Big Adventure + Sequel
I can see these two are getting some votes already, and deservedly so. They are quite simply some of the most memorable and endearing games I've ever played. However antiquated today, when they came out, there was simply nothing like them around, and for me at the time, it was without a doubt the most stimulating story I've ever played.
While hard to describe, it's a third person('ish) adventure game, set in a crazy unique world populated by crazy unique characters, that seems to have Mario'esque distress falling upon it from time to time. For me it was love at first playthrough, and I really couldn't imagine a better fit for GOG, than these two. They need to be rediscovered, and they need to be in my collection, that's for sure.
http://www.giantbomb.com/twinsens-odyssey/61-7623/
http://www.giantbomb.com/relentless-twinsens-adventure/61-11957/
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I was going to add Street Wars - Constructor Underworld, but I see GOG already beat me to it. Nice work!

Wasn't Silent Storm one of those games that got weird towards the end and started adding power armour or mechs?

Woooo yeah Rocket Jockey was boatloads of fun!!!!!!!! It's an instabuy for me when it's out. I don't think there's TCP/IP support for it though right? Might have to use VPN like Kali if that's the case.

Yeah, S2 was near perfection until the Panzerkleins (powered armor) popped up. They were fun as enemies, but sucked as weapons.

What about mecha-hitler at the end of wolfenstein?

I wonder if Hitler was trying to develop that....Brilliant.

I can't see how that would be more out of place than a psycho syringe throwing doctor, zombies, other high ranking nazi officials with miniguns on their hands, and those Hitler ghost clones, or whatever the hell that was.

Well as Germany was developing 1000/1500 ton tanks (yes, you read that right) and flying saucers at the end of the war, who knows what other weird things they had up their sleeve.

WOlfenstein was a sci fi/Fantasy from the start, and the Mech Hitler fit right in.
But for 90% of the game, Silent Storm was a fairly realistic World War 2 Tactical Game.And then, and the end, they introduced all the Klien Panzer Sci Fi crap. It just did not fit in with the rest of the game. It was out of place. ANd all the advertising for the game sold it as a realistic WW2 game.
Introudicng Sci FI elements at the last moment was simply a bad game design decision.

Like the others are saying, I thought that game was pretty zany even before that point. Did you miss the zombies with machineguns in their bellies?
Apart from the very subtle hints of conspiracy at the beginning, Silent Storm plays like a pretty straightforward WW2 game if you ask me, and I think most people agree that's where it truly shone.
It's a shame the developers didn't realize that, and it's a shame that the series essentially got shut down by Sentinels and the other cold war knockoff.

BADASS

Also, how come GOG has no Diggers?
It's another game that's been long since forgotten, but back when it came out it was just about some of the most fun you could have on a PC.
In short you control one of a number of fantasy races in a mad gem mining dash, digging tunnels through the ground, constructing mines, using drills, shovels and anything else you could think of to beat the competition and get those gems out.
One of the things I remember the best is the cartoony pixelgraphics which was very common around that time (Master of Orion, X-Com). Balance wise it may have been far from perfect, but most games of that time simply weren't, but we loved 'em for the addictive gameplay anyway.
I'd think it was abandonware by now, so I can see no reason why GOG wouldn't take a few hours and get this amazing game on their site.
http://www.mobygames.com/game/diggers
Seconded!