"True Internet Stealth: What Is It? Can It Be Achieved?" (http://lockdowncorp.com/stealth)
SANS look at some of the reconnaissance tools. (http://www.sans.org/rr/tools/tools.php)
SANS look at network scanning. (http://www.giac.org/practical/GSEC/Ronald_Black_GSEC.pdf)
SamSpade.org information-gathering web site. (http://www.samspade.org)
