Five, if you count Earthworm Jim 1 & 2 as one.
Also: Giants, TOCA 3, Sacrifice & MDK 2.
I've also got all three Fallout games, all the Unreal games, Earth 2150, Far Cry and Painkiller as boxed copies.

[url=]http://img516.imageshack.us/img516/5305/mygamesmyaccountgogcom1.jpg[/url]

My list of games:
TOCA 3
Colin McRae Rally 2005
UT 2004
Duke Nukem 3D
Redneck Rampage
The Guild Gold
Postal 2 Complete
FlatOut
1NSANE
Fallout 2
Chessmaster 9000
Apache vs Havoc
Far Cry
IL-2 Sturmovik: 1946
Prince of Persia: The Sands of Time
UT GOTY
Freespace 2

I think I'm up to 29, which is just under a quarter of the catalogue, last time I checked.
Next purchase: Obscure.
[EDIT] I bought Obscure and DN: Manhattan Project.[/EDIT]
Current List:
1. Arx Fatalis
2. Beneath a Steel Sky
3. Beyond Good and Evil
4. Darkstar One
5. Disciples Gold
6. Disciples 2 Gold
7. Duke Nukem 3D
7.5. Duke Nukem Manhattan Project
8. Fallout
9. Fallout 2
10. Fallout Tactics
11. Far Cry
12. Flat Out
13. Giants
14. In Cold Blood
15. Lure of the Temptress
16. MDK
17. MDK 2
17.5. Obscure
18. Oddworld Abe's Oddysee
19. Oddworld Abe's Exoddus
20. Prince of Persia: Sands of Time
21. Rise of the Triad
22. Sacred Gold
23. Sacrifice
24. Septerra Core
25. TeenAgent
26. Unreal Tournament 2004 EC
27. Warlords Battlecry
Yes, I have actually alphabetised them in the shelf view. Similarly, I've alphabetised my 200 strong collection of game discs. XD

I'm up to 28 paid so far 31 in total. :)
1) Simon the Sorcerer
2) Simon the Sorcerer 2
3) Fallout
4) Fallout 2
5) Sacred Gold
6) Stronghold
7) Disciples
8) Disciples 2
9) Broken Sword
10) Broken Sword 2
11) Giants
12) Lure of the Temptress
13) Beneath A Steel Sky
14) Earthworm Jim 1 & 2
15) Port Royale 2
16) Guild
17) Pure Pinball 2.0
18) Gothic
19) Runaway
20) Spell Force
21) Teen Agent
22) Septerra Core
23) Gorky 17
24) Beyond Good & Evil
25) Lionheart
26) Alien Nations
27) Sacrifice
28) Heroes of Might and Magic
29) Flatout
30) Toca: Race Driver 3
31) Insane
Any suggestions for any good games I don't have? :) I gotta increase my list.
*Sorry I lied! Just bought the Duke Nukem promo pack :)
30 Paid - 3 Free = 33.

4. (got one free during the closed(ish) beta when they did buy one get one free and another free from a competition on another site).
Fallout
Fallout 2 (my free game)
Freespace 2
MDK (got a gift code from comp and got this)

Happy with all of them. There are a few games I'd quite like to give a go, but currently, exams take priority...

5 as of now:
Beneath a Steel Sky (havent actually played it but hell it was free)
Beyond Good and Evil
Fallout
Painkiller: Black Edition
Prince of Persia: Sands of Time
All good purchases and as always you cant beat the price. More to come for sure

I'm addicted.
Bought:
MDK
MDK 2
Messiah
Sacrifice
Giants: Citizen Kabuto
In Cold Blood
Broken Sword II: The Smoking Mirror
Broken Sword: The Sleeping Dragon
Arx Fatalis
Darkstar One
Fallout
TOCA Race Driver 3
Flatout
Screamer
Downloaded for free:
Lure of the Temptress
Beneath a Steel Sky
Teen Agent
Of those, the only one I've actully played for more than half an hour so far is Fallout. It's not that I don't want to, just a question of setting aside time for it.

15
Darkstar One
Second Sight
Messiah
Painkiller
1nsane
Sacred Gold
Fallout
Fallout 2
Fallout Tactics
Shogo
Sacrifice
Freespace 2
Giants
Earthworm Jim 3D
Shattered Steel
Some bought just because I can't be bothered to look for my old CDs, others for being awesome back when I tried them (mainly Shattered Steel, the graphics have aged horribly) and some since they seem awesome.
There are a couple of games I'm still planning on getting.

My games on GoG:
Duke Nukem: Manhattan Project
Duke Nukem 3D: Atomic Edition
Runaway: A Road Adventure
Redneck Rampage Collection
Painkiller Black Edition
In Cold Blood
Broken Sword 3: The Sleeping Dragon
The Feeble Files
MDK
MDK 2
Messiah
Evolva
Freebies:
Teenagent
Beneath a Steel Sky
Lure of the Temptress
I actually haven't played these games almost at all. Just installed, checked if they are running, played for 30 minutes, burned install files on DVD.

5
Pure Pinball 2.0
Gothic 2
Postal 2
Toca 3
Big Race USA

Only been a GoG member for about a month. Still I already got:
Beneath a Steel Sky
Broken Sword: The Smoking Mirror
Duke Nukem 3D
Duke Nukem Manhattan Project
Fallout
Fallout 2
Fallout Tactics
Freespace 2
Gorky 17
Jagged Alliance 2
Jagged Alliance 2 Wildfire
Runaway a Road Adventure
Septerra Core
Spell Force
Stonekeep
The Guild
Toca Race Driver 3
And im already looking at a few more for the collection.

3 :)
TOCA 3
Flatout
Patrican 3
Next up is probably Colin McRae Rally 2005.

I have 17, but I was laid off in Feb, so I am not buying much of anything right now :-(

Joined not so long ago (few days). Have 3:
1. Toca 3 - from the promo
2. Painkiller Black - the best shooter ever
3. The Guild Gold - awesome game I bought yesterday, already spent 6hrs.