For years I used the free Avast AntiVirus program on my Mac (and PCs as well), and was pleased with its performance. Recently, however, the free version started doing small popups wanting me to upgrade or do something else. In short -- lots of nagging. Well, I decided to look around a bit at another free AV program, and settled on Avira, which I installed and am now using instead of Avast. If you're interested in reading more about Avira, following are two reviews:
Opinions vary on whether an antivirus program is needed on a Mac. I prefer to use one, especially one that is free and seems to be effective.