For those unfamiliar with Unix, dmesg is a log of the messages output by the kernel on boot as it initialises the machine's hardware, and possibly later as hardware devices such as USB sticks are connected or disconnected. For the purposes of this thread, only the boot messages are interesting, as later messages don't typically relate to what is normally attached to the machine.
If you are running a Unix-like operating system, such as Debian, OpenBSD or Mac OS X, you can obtain your dmesg simply by typing dmesg into a shell.
OpenBSD 4.9-current (GENERIC.MP) #138: Wed Jun 29 08:48:11 MDT 2011
deraadt@amd64.openbsd.org:/usr/src/sys/arch/amd64/compile/GENERIC.MP
real mem = 2136813568 (2037MB)
avail mem = 2065780736 (1970MB)
mainbus0 at root
bios0 at mainbus0: SMBIOS rev. 2.4 @ 0xf7950 (44 entries)
bios0: vendor Dell Inc. version "A08" date 04/02/2007
bios0: Dell Inc. MP061
acpi0 at bios0: rev 0
acpi0: sleep states S0 S3 S4 S5
acpi0: tables DSDT FACP HPET APIC MCFG SLIC BOOT SSDT
acpi0: wakeup devices LID_(S3) PBTN(S4) MBTN(S5) PCI0(S3) USB0(S0) USB1(S0) USB2(S0) USB3(S0) EHCI(S0) AZAL(S3) PCIE(S4) RP01(S3) RP02(S4) RP03(S3) RP04(S3) RP05(S3) RP06(S3)
acpitimer0 at acpi0: 3579545 Hz, 24 bits
acpihpet0 at acpi0: 14318179 Hz
acpimadt0 at acpi0 addr 0xfee00000: PC-AT compat
cpu0 at mainbus0: apid 0 (boot processor)
cpu0: Intel(R) Core(TM)2 CPU T7400 @ 2.16GHz, 2164.28 MHz
cpu0: FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,CFLUSH,DS,ACPI,MMX,FXSR,SSE,SSE2,SS,HTT,TM,SBF,SSE3,MWAIT,DS-CPL,VMX,EST,TM2,SSSE3,CX16,xTPR,PDCM,NXE,LONG
cpu0: 4MB 64b/line 16-way L2 cache
cpu0: apic clock running at 166MHz
cpu1 at mainbus0: apid 1 (application processor)
cpu1: Intel(R) Core(TM)2 CPU T7400 @ 2.16GHz, 2163.77 MHz
cpu1: FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,CFLUSH,DS,ACPI,MMX,FXSR,SSE,SSE2,SS,HTT,TM,SBF,SSE3,MWAIT,DS-CPL,VMX,EST,TM2,SSSE3,CX16,xTPR,PDCM,NXE,LONG
cpu1: 4MB 64b/line 16-way L2 cache
ioapic0 at mainbus0: apid 2 pa 0xfec00000, version 20, 24 pins
ioapic0: misconfigured as apic 0, remapped to apid 2
acpimcfg0 at acpi0 addr 0xf0000000, bus 0-63
acpiprt0 at acpi0: bus 0 (PCI0)
acpiprt1 at acpi0: bus -1 (AGP_)
acpiprt2 at acpi0: bus 3 (PCIE)
acpiprt3 at acpi0: bus 11 (RP01)
acpiprt4 at acpi0: bus -1 (RP02)
acpiprt5 at acpi0: bus -1 (RP03)
acpiprt6 at acpi0: bus 12 (RP04)
acpiprt7 at acpi0: bus -1 (RP05)
acpiprt8 at acpi0: bus -1 (RP06)
acpicpu0 at acpi0: C3, C2, C1, PSS
acpicpu1 at acpi0: C3, C2, C1, PSS
acpitz0 at acpi0: critical temperature is 126 degC
acpiac0 at acpi0: AC unit online
acpibat0 at acpi0: BAT0 not present
acpibtn0 at acpi0: LID_
acpibtn1 at acpi0: PBTN
acpibtn2 at acpi0: SBTN
acpivideo0 at acpi0: VID_
acpivideo1 at acpi0: VID_
acpivout0 at acpivideo1: LCD_
acpivideo2 at acpi0: VID2
cpu0: Enhanced SpeedStep 2163 MHz: speeds: 2167, 1667, 1333, 1000 MHz
pci0 at mainbus0 bus 0
pchb0 at pci0 dev 0 function 0 "Intel 82945GM Host" rev 0x03
vga1 at pci0 dev 2 function 0 "Intel 82945GM Video" rev 0x03
wsdisplay0 at vga1 mux 1: console (80x25, vt100 emulation)
wsdisplay0: screen 1-5 added (80x25, vt100 emulation)
intagp0 at vga1
agp0 at intagp0: aperture at 0xd0000000, size 0x10000000
inteldrm0 at vga1: apic 2 int 16
drm0 at inteldrm0
"Intel 82945GM Video" rev 0x03 at pci0 dev 2 function 1 not configured
azalia0 at pci0 dev 27 function 0 "Intel 82801GB HD Audio" rev 0x01: msi
azalia0: codecs: Sigmatel STAC9200, Conexant/0x2bfa, using Sigmatel STAC9200
audio0 at azalia0
ppb0 at pci0 dev 28 function 0 "Intel 82801GB PCIE" rev 0x01: msi
pci1 at ppb0 bus 11
ppb1 at pci0 dev 28 function 3 "Intel 82801GB PCIE" rev 0x01: msi
pci2 at ppb1 bus 12
athn0 at pci2 dev 0 function 0 "Atheros AR5418" rev 0x01: apic 2 int 19
athn0: MAC AR5418 rev 2, RF AR2122 (2T2R), ROM rev 4, address 00:1c:f0:fb:04:d7
uhci0 at pci0 dev 29 function 0 "Intel 82801GB USB" rev 0x01: apic 2 int 20
uhci1 at pci0 dev 29 function 1 "Intel 82801GB USB" rev 0x01: apic 2 int 21
uhci2 at pci0 dev 29 function 2 "Intel 82801GB USB" rev 0x01: apic 2 int 22
uhci3 at pci0 dev 29 function 3 "Intel 82801GB USB" rev 0x01: apic 2 int 23
ehci0 at pci0 dev 29 function 7 "Intel 82801GB USB" rev 0x01: apic 2 int 20
usb0 at ehci0: USB revision 2.0
uhub0 at usb0 "Intel EHCI root hub" rev 2.00/1.00 addr 1
ppb2 at pci0 dev 30 function 0 "Intel 82801BAM Hub-to-PCI" rev 0xe1
pci3 at ppb2 bus 3
bce0 at pci3 dev 0 function 0 "Broadcom BCM4401B1" rev 0x02: apic 2 int 17, address 00:18:8b:bd:20:1c
bmtphy0 at bce0 phy 1: BCM4401 10/100baseTX PHY, rev. 0
"Ricoh 5C832 Firewire" rev 0x00 at pci3 dev 1 function 0 not configured
sdhc0 at pci3 dev 1 function 1 "Ricoh 5C822 SD/MMC" rev 0x19: apic 2 int 18
sdmmc0 at sdhc0
"Ricoh 5C843 MMC" rev 0x01 at pci3 dev 1 function 2 not configured
"Ricoh 5C592 Memory Stick" rev 0x0a at pci3 dev 1 function 3 not configured
"Ricoh 5C852 xD" rev 0x05 at pci3 dev 1 function 4 not configured
pcib0 at pci0 dev 31 function 0 "Intel 82801GBM LPC" rev 0x01
pciide0 at pci0 dev 31 function 2 "Intel 82801GBM SATA" rev 0x01: DMA, channel 0 wired to compatibility, channel 1 wired to compatibility
wd0 at pciide0 channel 0 drive 0: <ST9160821AS>
wd0: 16-sector PIO, LBA48, 152627MB, 312581808 sectors
wd0(pciide0:0:0): using PIO mode 4, Ultra-DMA mode 6
atapiscsi0 at pciide0 channel 1 drive 0
scsibus0 at atapiscsi0: 2 targets
cd0 at scsibus0 targ 0 lun 0: <Optiarc, DVD+-RW AD-5540A, 102C> ATAPI 5/cdrom removable
cd0(pciide0:1:0): using PIO mode 4, Ultra-DMA mode 2
ichiic0 at pci0 dev 31 function 3 "Intel 82801GB SMBus" rev 0x01: apic 2 int 17
iic0 at ichiic0
spdmem0 at iic0 addr 0x50: 1GB DDR2 SDRAM non-parity PC2-5300CL5 SO-DIMM
spdmem1 at iic0 addr 0x52: 1GB DDR2 SDRAM non-parity PC2-5300CL5 SO-DIMM
usb1 at uhci0: USB revision 1.0
uhub1 at usb1 "Intel UHCI root hub" rev 1.00/1.00 addr 1
usb2 at uhci1: USB revision 1.0
uhub2 at usb2 "Intel UHCI root hub" rev 1.00/1.00 addr 1
usb3 at uhci2: USB revision 1.0
uhub3 at usb3 "Intel UHCI root hub" rev 1.00/1.00 addr 1
usb4 at uhci3: USB revision 1.0
uhub4 at usb4 "Intel UHCI root hub" rev 1.00/1.00 addr 1
isa0 at pcib0
isadma0 at isa0
pckbc0 at isa0 port 0x60/5
pckbd0 at pckbc0 (kbd slot)
pckbc0: using irq 1 for kbd slot
wskbd0 at pckbd0: console keyboard, using wsdisplay0
pcppi0 at isa0 port 0x61
spkr0 at pcppi0
mtrr: Pentium Pro MTRR support
uhub5 at uhub0 port 1 "Dell product 0xa005" rev 2.00/50.18 addr 2
uhub6 at uhub5 port 4 "Broadcom BCM2045B2" rev 2.00/1.00 addr 3
ugen0 at uhub6 port 1 "Broadcom Corp BCM2045" rev 2.00/1.00 addr 4
uhidev0 at uhub6 port 2 configuration 1 interface 0 "Broadcom product 0x4502" rev 2.00/1.00 addr 5
uhidev0: iclass 3/1
ukbd0 at uhidev0: 8 modifier keys, 6 key codes
wskbd1 at ukbd0 mux 1
wskbd1: connecting to wsdisplay0
uhidev1 at uhub6 port 3 configuration 1 interface 0 "Broadcom product 0x4503" rev 2.00/1.00 addr 6
uhidev1: iclass 3/1, 125 report ids
ums0 at uhidev1 reportid 2: 3 buttons
wsmouse0 at ums0 mux 0
uhid0 at uhidev1 reportid 125: input=2, output=0, feature=0
umass0 at uhub0 port 7 configuration 1 interface 0 "Maxtor Basics Desktop" rev 2.00/1.22 addr 7
umass0: using SCSI over Bulk-Only
scsibus1 at umass0: 2 targets, initiator 0
sd0 at scsibus1 targ 1 lun 0: <Maxtor, Basics Desktop, 0122> SCSI2 0/direct fixed serial.0d4974102HBAK7DK_
sd0: 476940MB, 512 bytes/sec, 976773168 sec total
uhidev2 at uhub1 port 2 configuration 1 interface 0 "Logitech Logitech USB Keyboard" rev 1.10/28.00 addr 2
uhidev2: iclass 3/1
ukbd1 at uhidev2: 8 modifier keys, 6 key codes
wskbd2 at ukbd1 mux 1
wskbd2: connecting to wsdisplay0
uhidev3 at uhub2 port 2 configuration 1 interface 0 "Logitech USB Optical Mouse" rev 2.00/54.00 addr 2
uhidev3: iclass 3/1
ums1 at uhidev3: 8 buttons, Z dir
wsmouse1 at ums1 mux 0
vscsi0 at root
scsibus2 at vscsi0: 256 targets
softraid0 at root
scsibus3 at softraid0: 256 targets
root on wd0a (35bf81d88edcf153.a) swap on wd0b dump on wd0b