The ClamAV was developed to be an open source (GPL) anti-virus toolkit, especially for e-mail scanning on mail gateways. It provides a number of utilities including a flexible and scalable multi-threaded daemon, a command line scanner and advanced tool for automatic database updates.

The core of the package is an anti-virus engine available in a form of shared library. Copy the virus database packages to the "database" directory inside the main ClamAV folder.

ClamAV Features:

POSIX compliant, portable
Fast scanning
Detects over 750.000 viruses, worms and trojans, including Microsoft Office macro viruses, mobile malware, and other threats
Built-in bytecode interpreter allows the ClamAV signature writers to create and distribute very complex detection routines and remotely enhance the scanner’s functionality

Scans within archives and compressed (also protects against archive bombs), built-in support includes:

Zip (including SFX)
RAR (including SFX)
7Zip
ARJ (including SFX)
Tar
CPIO
Gzip
Bzip2
MS OLE2
MS Cabinet Files (including SFX)
MS CHM (Compiled HTML)
MS SZDD compression format
BinHex
SIS (SymbianOS packages)
AutoIt
InstallShield

Supports Portable Executable (32/64-bit) & compressed or obfuscated with:

AsPack
UPX
FSG
Petite
PeSpin
NsPack
wwpack32
MEW
Upack
Y0da Cryptor
Supports ELF and Mach-O (both 32- and 64-bit)
Supports almost all mail formats

Support for other special formats includes:
HTML
RTF
PDF
Files encrypted with CryptFF and ScrEnc
uuencode
TNEF (winmail.dat)
Advanced database updater with support for scripted updates, digital signatures and DNS based database version queries