pcguide.comis a good source of detailed as well as comprehensive information about nearly every part of the PC
ePanorama.netcollected articles and links about nearly every hardware.
7Voltsis the place to go if you want to enhance your PC. There are plenty ideas about reducing noise, improving cooling or wiring. And it also shows that computer cases do not need to be boring boxes.
www.hut.fi/Misc/Electronics is an archive with plenty of documents and assembly instructions of electronic components of all kind
RepairFAQ.orgdoes not only describe several important PC components...
Hardware Bookoffers a large list of cable and connector pinouts.
Interfacing the PCAs the name says a useful page especially for electronic experts
Superputdescribes how to use any simple VGA card combined with a small, cheap electronic circuit in order to display an X server on a TV screen. There are also some other ideas and charts.
Soundcard specific links
Grassomusiccontains several add-ons and modifications for adding connectors (analog, MIDI, SPDIF), increasing sound quality as well as other useful stuff. There are a lot of modifications for SB32 / 64 and SB Live, too. General understanding of electronic circuits is recommended in order to benefit from this site.