Hmm... Here's what I got in no particular order:
1) Beneath A Steel Sky (free)
2) Lure of the Temptress (free)
3) Teenagent (free)
4) Broken Sword 2
5) Unreal Tournament 2004
6) Duke Nukem 3D: Atomic Edition
7) Duke Nukem: Manhattan Project
8) Ghost Master
9) Chessmaster 9000

Picked up Duke Nukem 3d the other day and am looking forward to purchasing Seven Kingdoms in the future.
Already own all the Fallouts on disc.

Update: almost six games.
On another note, I would own another game, but a coupon error on GoG stopped me from buying a new game.
I inputted a one-time use coupon. I backed up one window to update my cart. My coupon got removed from the checkout page. I tried to input the coupon code again. Now, my coupon code is no longer valid, even though, I never finished making my purchase.

12.

You should contact support about this.

That's dedication, wow. If I had plenty of money, I'd do the same. Sadly I'm broke. Before I went broke I managed to get 6 (not including free). Just one more and I'll have another shelf XD.
Favorites are: CMR2005 and Arx Fatalis

Wow! This thread is still alive! And got mentioned on GoG's News page. Coolness!
Adding on to my list, I now have purchased (and some free ones):
5) Megarace 1+2
6) Fallout
7) Fallout Tactics
8) Teenagent
9) Lure Of The Temptress
10) Beneath A Steel Sky
11) Jagged Alliance 2
12) I.G.I. 2: Covert Strike
13) Conquest Of The New World
Looking at this list now, what was I thinking!?

I would like to add my favourites are Sacred Gold, TOCA 3, Fallout (haven't played 2 yet), Duke 3D, and MDK. So much joy for so little money. :)
Course I haven't played most of them. :P

So far I've only purchased Fallout 1 & 2 from GOG. There are a few games I have my eye on though, and a few I already own.

Only one for now: Heroes of Might and Magic - because I couldn't find anywhere this FIRST incarnation of the beloved turn-based game.
I love them all, BTW, except the HOMM V series. The 3D view is too confusing for how my brain is built. Makes me puke.
I'm looking forward for more great DOS games! Dynablast, Micromachines, some other older turn-based strategy games - I'd buy them without hesitating. Civilization 1....

I know what you mean =D
Funny how the numbers are different between the first and the second part in this thread. Nice to see I'm not the only one addicted to buying games here =)
Actually, I'm not that bad. I have 12 right now, plan to buy gothic1+2 or Celtic Kings: Rage of War next. Have my eye on The Guild as well.
-fallout 1, 2, tactics
-sacrifice (even though I own the disc)
-Giants: Citizen Kabuto
-Freespace 1, 2
-Sacred
-Disciples 2
-Arx Fatalis
-Stonekeep
-Beyond Good and Evil

[url=http://petricek.org:81/gog-com_shelf.png]My shelf[/url]

5 already... not really played them actually ;)
- fallout 1
- fallout 2
- fallout tactics
- jagged allience 2
- original war

Only 5 but no time to play them. :(