I've written a small Perl utility that takes dmesg output on standard input and produces a human-friendly tree of devices on standard output. Since it accepts any dmesg output, you can use it to produce a tree of hardware on someone else's system as well as examine your own.
As an example, this is what it looks like on my Lemote Yeeloong:
$ dmesg | devtree
root
|-mainbus0 (Lemote Yeeloong)
| |-apm0
| |-bonito0 (memory and PCI-X controller, rev 1)
| | |-pci0 (bus 0)
| | |-auglx0 (AMD CS5536 Audio)
| | | |-audio0
| | |-ehci0 (NEC USB)
| | | |-usb0 (USB revision 2.0)
| | | |-uhub0 (NEC EHCI root hub)
| | |-ehci1 (AMD CS5536 USB)
| | | |-usb1 (USB revision 2.0)
| | | |-uhub1 (AMD EHCI root hub)
| | | |-umass0 (Generic USB2.0-CRW)
| | | | |-scsibus0 (2 targets, initiator 0)
| | | | |-sd0 (Generic-, Multi-Card, 1.00)
| | | |-urtw0 (Realtek RTL8187B)
| | |-glxpcib0 (AMD CS5536 ISA)
| | | |-glxclk0 (clock, prof)
| | | |-gpio1 (32 pins)
| | | |-iic (not configured)
| | | |-isa0
| | | |-mcclock0 (port 0x70/2: mc146818 or compatible)
| | | |-pckbc0 (port 0x60/5)
| | | | |-pckbd0 (kbd slot)
| | | | | |-wskbd0 (console keyboard, using wsdisplay0)
| | | | |-pms0 (aux slot)
| | | | |-wsmouse0 (mux 0)
| | | |-ykbec0 (port 0x381/3)
| | |-ohci0 (NEC USB)
| | | |-usb2 (USB revision 1.0)
| | | |-uhub2 (NEC OHCI root hub)
| | |-ohci1 (AMD CS5536 USB)
| | | |-usb3 (USB revision 1.0)
| | | |-uhub3 (AMD OHCI root hub)
| | |-pciide0 (AMD CS5536 IDE)
| | | |-wd0 (Corsair Force 3 SSD)
| | |-rl0 (Realtek 8139)
| | | |-rlphy0 (phy 0: RTL internal PHY)
| | |-smfb0 (Silicon Motion LynxEM+)
| | |-wsdisplay0 (std, vt100 emulation)
| |-cpu0 (STC Loongson2F CPU 796 MHz, STC Loongson2F FPU)
|-softraid0
| |-scsibus2 (256 targets)
|-vscsi0
|-scsibus1 (256 targets)
Source and a man page are available under the WTFPL from git://git.sjm.so/openbsd-goodies.git (or view it on GitWeb) .