Nikto(one of the best web server scanners)
http://www.cirt.net/nikto/nikto-1.36.tar.bz2

NMAP (excellent security tool - port scanner, OS detector e.t.c.)
Here's its download page:
http://ftp.iasi.roedu.net/mirrors/download.insecure.org/nmap/nmap_download.html

NetCat
Another security tool - port scanning, port listening e.t.c. It's even called TCP/IP Swiss Army Knife.
Windows version:
http://www.vulnwatch.org/netcat/nc111nt.zip
Linux version:
http://www.vulnwatch.org/netcat/nc110.tgz

Wireshark - the best port sniffer
Windows version:
http://wireshark.dsmirror.nl/download/win32/wireshark-setup-0.99.6a.exe
Linux source-code:
http://www.wireshark.org/download/src/wireshark-0.99.6.tar.gz

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