PCI Configuration Register. A 256 byte block of data used to configure each PCI device. Generally each device is configured by the BIOS and Operating System, and the PCR is completely undocumented.
The PCRs can be edited by a program called WPCREdit, easily found with a google search. Of most interest are the north and south bridge devices, the editing of which can enable all kinds of features not available in the BIOS configuration. Changes that can be effected include playing with PCI timing/backoff behaviour, changing memory/cpu frequency on the fly, and messing with AGP settings. .PCR files, which describe the function of each register are easily found for chipsets by intel, amd and via, and even for some soundcards.
Changes to the registers last until the machine is reset, so (many websites claim), it is impossible to cause lasting damage to your computer.1 The program WPCRSet can be run at startup to make desired changes permanent.
1 - This is a lie. Messing with the PCI bus settings while the OS is running can cause PCI devices to malfunction. If your IDE or SCSI host controller malfunctions, it can cause widespread data loss. Yesterday, in an attempt to make my PCI bus more soundcard-friendly, I managed to scribble zeros over nine partitions, spread over two drives, and leave the computer completely unbootable. Be warned.