Craig Hart's PCI Page Craig has the ultimative list of all PCI devices and vendors. Support him by sending him inf files from drivers of hanrdware which is not known yet.
PCIScope A better tool to scan the PCI bus. An evaluation version is downloadable. Works with Windows 9x/2000 and XP.
PCI SIG The PCI Special Interest Group is the main forum of PCI Interest. They make the standards and guidelines for PCI. They stay near Intel round the corner :-)
Ralf Brown Ralf Browns PCI Stuff. He has many thing round the hardware.Further you may get complete DOS Interrupt Lists. The INT $1A-List for PCI BIOS can downloaded there too.
Kolter Electronic Kolter Electronic Homepage. Here are mor links and a prototype board using Lattice 103x.
Internet PCI Product Database Another very actual database (Internet only) for PCI, AGP and Cardbus devices.

More Links:

PCI-Viewer for Windows 9x / NT

General PCI & PNP

Signals at PCI-bus: Pinout

PCI background informations

Good PCI- and BIOS-links

PCI- driver development software

Linux HOWTO for PCI devices

32-bit Compiler (freeware)