A program which does phrase based filtering. Useful for schools and such like that can't afford to have children viewing pornography.
It has a very quirky license being partially GPL and partially proprietary, although it's got the OpenSource foundations stamp of approval, and even RMS seems to like it.
cat > /etc/logrotate.d/dansguardian << EOF /var/log/dansguardian/access.log {
rotate 31 daily prerotate
/etc/init.d/dansguardian stop || true
endscript postrotate
sleep 2 && /etc/init.d/dansguardian start || \ (sleep 2; /etc/init.d/dansguardian start)
endscript
} EOF
4 pages link to DansGuardian: