Here's mine.

OS:
base: Windows 7 Ultimate 64-bit
other: currently none

Hardware:

Motherboard : Gigabyte GA-MA780G-UD3H
Processor : AMD Phenom II 920 AM2+ BOX
Memory : GEIL Ultra Plus 4096Mb DDR2 1066MHz KIT
Video card : MSI R4850-2D512-OC PCIE
PSU :Gigabyte Superb 460W
HDD : Samsung 1000GB 7200rpm 32MB SATA2
Monitor: LG Flatron F70B ( looking for a new monitor )

Post yours.

OS: Windows 7 x64 + few VMs (XP, Ubuntu).

Hardware: link.

OS;
tinyXP

Hardware

Mobo: Some Gigabyte.
CPU: AMD 3200+
Memory: Generic 2GB ddr1
GPU: Sapphire 4850
PSU: 430W too lazy to check what brand
HDD: Two but only one connected. 120GB IDE HDD and 1T Spinpoint F1 (one of the best hardware purchases ever)
Monitor: 20in LG
Keyboard and mouse: Logitech Wireless
Speakers: 5.1 Logitech
Printer: Canon MP180

OS X 10.6 Snow Leopard

Hardware:

Model Name: MacBook Pro
Processor Name: Intel Core 2 Duo
Processor Speed: 2.4 GHz
Number Of Processors: 1
Total Number Of Cores: 2
Memory: 3 GB
HD Capacity: 500 GB

MSI Wind U100 netbook.
Windows XP
2GB RAM
Intel Atom N270 1.6GHz
Intel GMA 950

This isn't the computer I use to play games on, but it is the one I use most frequently (especially for GOG). Plays some newer games as well.

The PC I use for games:

Windows 7 Home Premium 64bit
AMD Phenom 9750 Quad-Core CPU @ 2.40GHz
8GB RAM
NVIDIA GeForce 9500 GS
700GB HD
LG Flatron monitor (unsure of the detailed specs)

Rig 1:

OS: Windows 7 Professional 64 bit
Mobo: Gigabyte EX58-UD5
CPU: Intel i7 920
Memory: GSkill 3x2GB Triple Channel RAM kit
Graphics: EVGA GTX 275
SSD: OCZ Vertex 60 GB (OS drive)
HDD: Western Digital 1.5 TB and WD 640 GB
Monitor: Philips Brilliance 240PW
PSU: Corsair HX850

Rig 2:

OS: Windows XP Home SP3 32 bit
Mobo: Gigabyte P35-DS3L
CPU: Intel C2D E6750
Memory: 2x2GB Kit Mushkin Redline
Graphics: EVGA GTX 260
HDD: Seagate Barracuda 160 GB
HDD: WD 250 GB
HDD: WD 500 GB
Monitor: LG Flatron W2242T
PSU: PC Power and Cooling Silencer 750W

Rig 3:

OS: Windows XP Home SP3 32 bit/ Ubuntu
Mobo: ASUS A8N SLI
CPU: AMD Athlon XP 3200+
Memory: 4x512 MB Crucial
Graphics: EVGA 8800 GT
HDD: Seagate Barracuda 250 GB
Monitor: Samsung SyncMaster 940b
PSU: Antec 450W

OS: Windows 7 Pro 64-Bit
Mobo: Asus P5E (Flashed with Rampage Formula Bios)
CPU: Intel Q6600 @ 3.2Ghz
Memory: 8GB OCZ Reaper 800Mhz DDR2
Graphics: MSI HD4850 512MB
Monitor: Samsung SyncMaster 23" 1920x1080
PSU: OCZ 780W ModXtreem
Sound: Creative X-Fi Titanium Fatal1ty Champion series
Drives: LG Blu/HD-DVD combo, 2x 1TB Samsung F1, 1 x WD 500GB Caviar.
Cooler: Thermalright TRUE 120 with two Xilence fans
Case: Antec 1200

Edit: Adding the bling, aaah yeah.

Windows 7 64

Phenom II 940
4GB DDR2
Geforce GTX 260
Xonar DS
500GB + 150GB HD (probably the next upgrade).
Dell 22" e-IPS 2209WA

Desktop:

HP Pavilion Elite m9458f

Primary OS: Windows 7 Pro x64
Secondary OS: Fedora 13 x64

Mobo: Asus IPIBL-LB
CPU: Core 2 Quad Q6600 2.4GHz (8mb L2 cache)
Memory: 4 X 1GB DDR2 PC26500 (800 MHz)
Graphics: ATI Radeon HD3650 (512MB)
HDD: 500GB 7200RPM SATA
Optical: LG GBC H20-L Blu-Ray Rom
Monitor: DELL S2209W 22" 1080p


Notebook:

Toshiba M70-SR3

OS: Windows XP Home SP3
CPU: Pentium M 1.73GHz (with centrino tech) 2MB L2 cache
Memory: 1.5GB DDR2 PC4200 (533MHz)
Graphics: 128MB GMA900 intergated shared vram (awful)
HDD: 100Gb SATA 5400RPM
Optical: Matsushita DVD-RAM drive (can no longer read cd's)
15.4" screen 1280 X 800

Despite the awful video card, it was above average overall laptop in '05 when I got it as not all laptops used DDR2 ram then and most used PATA 4200RPM hard drives at 40-60GB. It's still very useful today. 2 MB L2 cache is also very good for a single core.

Ok on my main computer :

Primary OS : Windows 7 Ultimate 64
Secondary OS : Windows 7 Ultimate 64 (this one for games)

Mobo: Asus Rampage Extreme III
CPU: Intel Core i7 960 @ 3.20GHz
Memory: 12.0GB Triple-Channel DDR3
Graphics: ATI Radeon HD5780
Optical: SAMSUNG DVDWBD SH-B083L

HDDs:
117GB Corsair SSD-F120GB2
977GB Western Digital WDC WD1002FAEX-00Z3A0

Monitors:
Eizo HD2442W @ 1920x1200
Viewsonic VP191s @ 1280x1024

Tower:
Windows 7 Ultimate x64
Asus M4A785T-M
AMD Athlon II X2 250 @ 3 GHz
4x2 GB DDR3
Sapphire ATI Radeon 5750, 1 GB GDDR5
Samsung Spinpoint 750 GB
3x Seagate Barracuda 320 GB
2x Western Digital Caviar Green 2 TB
Logitech G11
Logitech M705
2x LG Flatron L2000CN @1600x1200 (both on DVI)

Media centre:
Mac Mini
Mac OS X 10.6 Snow Leopard
Intel Core2Duo @ 1.83 GHz
1 GB DDR2
Intel Retardism(tm) graphics
Hitatchi Travelstar 80 GB
LG Flatron L1752TQ @ 1280x1024 (DVI)

Laptop 1:
Lenovo Thinkpad R500
Windows 7 Ultimate x64
Intel Core2Duo P8700 @ 2.53 GHz
2GB DDR3
ATI Mobile Radeon 3400-something, 128 MB
Fujitsu 320 GB
Built-in 15.4-inch screen @ 1680x1050

Laptop 2:
IBM Thinkpad T21
Windows 2000 Professional
Intel Pentium 3 @ 600 MHz
256 MB PC100
S3 Savage/IX 1014, 8MB
IBM Travelstar 20 GB
LG Flatron L2000CN @ 1024x768 (VGA)
battery doesn't hold charge and built-in screen is broken

Server:
Mac Mini
Mac OS X 10.5 Leopard Server
Freescale PowerPC G4 @ 1.25 GHz
1 GB DDR
ATI Radeon 9200 @ 32 MB
Samsung Spinpoint M 120 GB
LG Flatron L2000CN @ 1600x1200 (VGA)

Nostalgia 1:
Macintosh Performa 6200
Mac OS 9.1
PowerPC 603 @ 75 MHz
64 MB RAM
1 MB VRAM
2 GB HDD
Apple Multiple Scan 14 @ 640x480, 16-bit colour (can handle 832x624, 8-bit)

Nostalgia 2:
Macintosh Performa 475
Mac OS 8.1
Motorola 68LC040 @ 25 MHz
36 MB RAM (4 soldered onto the mobo, and a 32 MB stick in the only slot)
512 KB VRAM
4 GB HDD
Apple Multiple Scan 14 @ 640x480, 8-bit colour

And then there are two laptops that I haven't messed with in a while, one AST Ascentia 950N with a Pentium running Windows 95 (if I remember correctly), and one Victor V386PX powered by a mighty 386 as the model name suggests.

Edit because this threads re-sparked the idea of fixing the old laptops:

AST Ascentia 950N:
Windows 95A
Pentium @ 90 MHz
16 MB RAM
800 MB HDD
Built-in TFT screen @ 800x600, 8-bit colour

..the Victor will be added when I've fixed a new CMOS battery to replace the weird one it's got now and figured out more of what hardware it's got. What I do know is there's a 3.5" HDD (yes, not a small and light 2.5" drive), and a standard 3.5" floppy drive more often found in desktop machines, not one of those thin ones found in laptops.

Main:
OS: Windows 7 Home Premium 64-bit
Hardware:
Motherboard : Gigabyte GA-MA770T-UD3P
Processor : AMD Phenom II x4 955
Memory : 4GB DDR3-1333
Video card : Asus HD5750
PSU : OCZ Fatal1ty 550W
HDD : 640 GB 7200rpm 32MB SATA2
CPU-Cooler: OCZ Gladiator Max
DVD-writer: Sony SATA2
Case: Coolermaster Gladiator 600
Monitor: Medion 17" (hopefully soon to be replace by 22" widescreen)

Oh yeah, the missus has an Medion P4 with 1,25GB memory, 100GB HDD, a Geforce 4, and a 19" CRT, running Windows XP. (Still playing GOG-games!)

Gaming rig that's thousands of miles away:

Windows 7 Ultimate 64-bit
ASUS Striker Extreme
Intel Core 2 Extreme X6800 2.93 GHz
EVGA GTX 295 1792 MB
4 GB DDR2 RAM
2 x Dell 2007WFP 20-inch monitors (I use one but two if I feel like it)
Samsung DL drive thing
Western Digital Caviar Black 2 X 500GBs in RAID 0
Antec 900

Laptop I use right now:

Acer Aspire 4736ZG

Intel Pentirum T4400 2.2 GHz
nVidia G105M 512 MB
2 GB DDR2 RAM
14 inch LCD
320 GB Hard Drive
Some DL drive or whatever

I'm thinking of upgrading my laptop CPU (yes, really). Hmm.

Acer Aspire Revo R3610 - This is a dual core Intel Atom nettop using nvidia ION.

As for the OS, Ubuntu (currently 11.04 pre-alpha)

This is one reason I love GOG.com, my machine may not be powerful by todays standards but it is quiet, plenty powerful for most of my uses and cheap. Yet all the games I get here tend to run great on the hardware. I do wish for better testing gone with wine and dosbox so we Linux users could be sure the games we buy will run. That being said I tend to buy games here just to support the concept, I have not played most of my, growing collection, yet.

OS : Vista Home Premium 32 Bit
Motherboard : Gigabyte EP35-DS3R
Processor : Intel Core 2 Duo 3.0 GHz
Memory : 4 GB OCZ DDR2 1066 MHz
Graphics : XFX manufactured GeForce 8800 GTS 512
PSU : Corsair TX650W
HDD : Seagate 500 GB, external : Western Digital 620 GB
Monitor : Dell E228WFP 22"
Case : Coolermaster CM690 mid tower