I have yet to have much of a need for antivirus software under Linux, or well more like BSD in my case, but anyway either platform is relatively resistant to popular attack vectors associated with most high-profile exploits targeting the Windows platform.
If I were to deploy Linux in a mission critical role, such as a server-class system, then I might be more inclined to consider antivirus measures. Otherwise, in a typical PC role, basic Linux security can be shored up decently with a good firewall configuration, regularly checking for software updates, and applying common sense to basic operating procedures like dealing with root accounts and similar fundemental UNIX-style issues. Many modern distributions offer graphical interface utilities to configure such network security parameters.
If you want to occasionally scan a Linux system, then the open source ClamAV project offers a console interface for antivirus scanning.