I will second Worms Armageddon, just because that is just plain awesome.
Now, here is my list of games, and my reasoning:
ALL the LucasArts games. My reasoning here is simple: More money for LA to do more awesome retro game rereleases & the like.
RollerCoaster Tycoon 1 & 2. These would be nice to play for those of us with older computers.
ALL the games from the "white whales" (e.g. big-name publishers that haven't signed with GOG yet).
Then I'll be happy.
BJ

Abandonware is at least in the grayzone of legality if not outright illegal. Still, I cherish them for what the do.
They are far from being just another pirates torrent-site. I'm not aware of any site dedicated to Abandonware who doesn't take games off, if they can get bought on some regular stores. In most cases, they even add links to places where you can get them legally.
The main goal of Abandonware is and will always be, the preservation of these games. Many games would have by now been plain lost to history if it wasn't for these guys. Heck, the basic goal of GOG is actually just the next step of Abandonware sites, keeping and making these games available, but in a legal way.
With that being said, I hope the industry will someday recognise and value what they're trying to do. A history lesson on computer- / videogames is just THAT much better if you can actually get back to these games, instead of reading some old reviews or watch some youtube videos.

You better go tell EA to take C&C:Tiberium Dawn and Red Alert off their FTP servers then, as they're offering them for free as ISOs of the 2 discs.
Official C&C forums post
Looks like they added Covert Ops addon for C&C as free download too ^_^.
They're just intent on ruining the £35 asking price for TFD pack aren't they? Giving away the best games for free and making you pay for the crap like Generals :dry:

Hmm well maybe I am wrong. But why offer games for free and sell them at the same time. I don't get it. And hey, i liked Generals.

LOL
That is all I have to add to this.

They started selling TFD long before they released even the first C&C as abandonware. The value of the pack may have diminished as a result, but there are still several games and expansions that aren't free in there.

Totally agree on Heretic and Hexen. I would add Blood, which is a real pain to make work with DosBox. That would be an instabuy from me if GOG team can make it run smoothly on modern computers !

Microsoft games, but the Close Combat series
They were AWESOME-I stil have my copy of a bridge too far

Nocturne
Hexen
Heretic
Clive Barker's Undying
Return to Castle Wolfenstein
Shadow of the Comet
Call of Cthulhu: Dark Corners of the Earth
Realms of the Haunting
Alone in the Dark (the version with cdrom music)

Yeah some games don't work properly with DOSbox, no matter what. Blood really should be here given that Monolith are.

Microsoft would probably object to GOG adding Windows XP compatibility to their games.

There are many Julian Gollop games that would make fine GOGs.

Oooh, good list! I seem to remember Nocturne having compatibility issues, even back in the day. So I doubt that would be an easy one to convert.

The Neverhood looks bloody amazing! That's exactly the kind of game that belongs here. I'd definitely snap that one up.

I'd love to see any of these games here=
Toonstruck
Black Dalia
Full Throtle
Zork Grand Inquisiter
Anachronox

A good selection. And not one of them I own. I wasn't even aware there were any (comparatively) recent Zork games. So I'd certainly welcome those.

Dungeon Lords? Never got to play my copy since computer sucked, new one can run it.

Apparently it's not so bad once fully patched up. Still I bet that GOG would have their hands full with that one.