Chkrootkit / Rootkit Hunter / Aide / Samhain / Snort / Integrit / Osiris or tripwire

http://www.la-samhna.de/samhain/

http://integrit.sourceforge.net/texinfo/integrit.html

The Samhain host-based intrusion detection system (HIDS) provides file integrity checking and log file monitoring/analysis, as well as rootkit detection, port monitoring, detection of rogue SUID executables, and hidden processes.