ClamAV (Clam anti-virus) is a suite of programs designed to detect malware, viruses, trojans, and other threats. It be run from the command line or as a daemon, and can integrate with mail servers to scan email attachments. It is open source and published under the GPL.
From the apt-cache description:
ClamAV is an open source (GPL) antivirus toolkit, which is designed to detect trojans, viruses, malware and other malicious threats. The package provides a flexible and scalable multi-threaded daemon in the clamav-daemon package, a command-line scanner in the clamav package, and a tool for automatic updating via the Internet in the clamav-freshclam package. The programs are based on libclamav6, which can be used by other software. . This package contains the daemon and its command line interface, featuring: - fast, multi-threaded daemon; - easy integration with MTA's; - support for on-access scanning; - remote scanning; - able to be run supervised by daemon.