Sendmail: http://www.sendmail.org/
Latest version and documentation of Smail: http://www.smail.org/
Qmail source, documentation and supporting software: http://www.qmail.org/
Apache web server, related projects and documentation: http://www.apache.org/
Squid proxy server: http://www.nlanr.net/squid/
Sunsite: http://metalab.unc.edu/, ftp://metalab.unc.edu/
tproxy package: ftp://ftp.nlc.net.au/pub/linux/www/