I am trying to compile a list of tools used by adminstrators for security and system tools. I am looking for some help.
Please add to the list and list urls for more thank you.

PortSentry (http://www.psionic.com/products/portsentry.html/)
Tripwire (http://www.tripwire.com/)
Chkrootkit (http://www.chkrootkit.org)
Nmap (http://www.insecure.org/nmap/)
Snort (http://www.snort.org)

http://www.insecure.org/tools.html --- Good start...does anyone have any others they like or use?

Also from Psionic, I like LogSentry (formerly LogCheck):

http://www.psionic.com/products/logsentry.html

I use these a lot:

nmap (for scanning my network)
ncat (for testing stuff)
tcpdump (for watching packets)
fragrouter (for generating fragmented packets)
ipsorcery (for generating packets)
man (for reading man pages, of course)
less or vim (for reviewing code)
msie (for browsing security web sites)
ttysnoop (for snooping on ttys)

:)