I found it difficult to obtain the Ridge Racer operation manual online, until I stumbled across this following link.
It may be well known to people here, however it is very useful so here you go:
Nowhere near as many, I'm sure, but there's also:
Some manufacturers have the manuals on their sites:
Some pretty hard to obtain Sega manuals in PDF form here. Snap them up and store 'em!
Dipswitches, pinouts, manuals and more:
http://www.basementarcade.com/ scroll down
Thanks to Retro, Unclejun, trmatthe on Jammaplus for their contributions.