Recommended free virus software

No, it isn't necessary at all. If it will make you feel better to have something, the only one I can recommend at all is ClamXav, which does happen to be free. Avoid the commercial ones.

Mac OS X versions 10.6.7 and later have built-in detection of known Mac malware in downloaded files. The recognition database is automatically updated once a day. That's all the automated protection there is against malware; however, you shouldn't rely on it, because the attackers are always at least a day ahead of the defenders..

The only effective defense against malware is your own intelligence. All known Mac malware takes the form of trojans that can only operate if the victim is duped into running them. If you're smarter than the malware attacker thinks you are, you won't be duped. That means, primarily, that you never install software from an untrustworthy source. Any website that prompts you to install software, such as a “plug-in,” that comes from that same site is untrustworthy. If software has a corporate brand, such as Adobe Flash, you always download it directly from the original source, never from an intermediary.

Never install any commercial "anti-virus" products for the Mac, as they're all much more harmful than the mostly imaginary threat they purport to defend against. If you need to be able to detect Windows malware in your files, use ClamXav -- nothing else.