Clamav, Chkrootkit, Portsentyr, Tripwire

Clamav:

ClamAV is an open source (GPL) antivirus engine designed for detecting Trojans, viruses, malware and other malicious threats. It is the de facto standard for mail gateway scanning. It provides a high performance mutli-threaded scanning daemon, command line utilities for on demand file scanning, and an intelligent tool for automatic signature updates. The core ClamAV library provides numerous file format detection mechanisms, file unpacking support, archive support, and multiple signature languages for detecting threats. The core ClamAV library is utilized in Immunet 3.0, powered by ClamAV, which is a fast, fully featured Desktop AV solution for Windows.

Chkrootkit:
chkrootkit (Check Rootkit) is a common Unix-based program intended to help system administrators check their system for known rootkits. It is a shell script using common UNIX/Linux tools like the strings and grep commands to search core system programs for signatures and for comparing a traversal of the /proc filesystem with the output of the ps (process status) command to look for discrepancies.

http://en.wikipedia.org/wiki/Chkrootkit

Portsentry

Tripwire