Free third-party antivirus is often a tradeoff. You receive only basic protection—a term that means different things to different companies—as well as semi-regular “encouragement” to upgrade to a paid suite.
AVG AntiVirus Free for Mac offers very basic protection, but isn’t too terrible about encouraging upgrades. The suite protects against viruses, secures your desktop email, your web browsing, and that’s it. Those are the essentials, however, which go a long way to keeping your computer safe.