The web security tools

netcat - a featured networking utility which reads and writes data across
network connections, using the TCP/IP protocol.

http://netcat.sourceforge.net/

nmap - a free and open source (license) utility for network exploration or
security auditing

http://nmap.org/

paros - a web proxy
for people who need to evaluate the security of their web
applications

http://www.parosproxy.org/index.shtml