Here are some useful Z related sites on the web:

http://zcar.com                    Excellent Forums - A gathering place for Z fanatics to share info

http://www.zdriver.com            Another superb source for Z related info. 

http://www.zcarparts.com        Motorsport Auto - carries a full line of Z products.  Sometimes prone to questionable reliability in proper part delivery and availability.  Nice free catalog, though..

http://members.aol.com/_ht_a/ohiozclub/ohioz.html        Z Car Club of Ohio

http://pub28.ezboard.com/bdatsun280zx        A message board dedicated to the 79-83 ZX models

http://www.mame.mu.oz.au/salman/zpage.html        A great compilation of Z history and technical info

http://ideal-z.com/        Source for some aftermarket parts, tips, and Factory Service Manuals on CDROM

http://www.zspecialties.com/        This is where I purchased the GM alternator upgrade for my car

http://members.xoom.com/midohiosc/        A site for central Ohio import car enthusiasts

More to come..