If you're talking about vuln scanner, I'd suggest :
nessus (my favourite one), or even whisker (written in Perl)...
If you're talking about detecting intrusion, a search with IDS as keyword will bring you tons of results :
Portsentry, Snort, Hostsentry, NFR, Tripwire, Toby...
Anyway a list of excellent security tools is available here (thanks to fyodor).
"Only Bad Coders Code Badly In Perl" (OBC2BIP)
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ATS,
There are a few, check out freshmeat
for general and a specific one would be
saint
-derby | [reply] |