Free and Lame:
http://www.csm.astate.edu/~wpaulsen/chess/chess.htm
Free and Useful:
http://www.eudesign.com/chessops/ch-clear.htm
http://www.chessgames.com/perl/explorer
PayForOpeningBooks:
http://www.chessgames.com/perl/explorer
www.chesspublishing.com
PossiblyIllegal:
http://www.geocities.com/chess_sharewith_us/
DontKnowWhatToThink:
http://www.chesslab.com/PositionSearch.html
just to explain :
opening books are a serious part of correspondence chess.
lets let everyone have them.
use them if you want.
skip them if you want.
but if your opponent, no matter how low or high rated, uses these moves then that is your problem!
p.s. i normally dont use any ... i like it that way ... and i like it if you use them to learn me a lesson or two.
www.chessbase.com
Im sure there is a database there. I remember it not quite working when I tried it last, probably just my pc.
I use chessgames mainly along with my own databases.
I wonder if we could compile a mega database between RHP users, just all adding our own databases to it and then host it on the net so each of us could download it.
Just a thought.