While it's unlikely you'll ever run into malware for the Mac, you may want to consider an antivirus tool anyway—if not to protect yourself, but to protect your Windows-using friends from any malware you may inadvertently send their way. We think that Sophos Anti-Virus for Mac is the best choice, and it's free.
Update: Our previous recommendation for Mac Antivirus was "nothing." We know many of you choose to use nothing, but we also know you don't come to the App Directory to not get any suggestions at all. Besides, malware is starting to become a bit more prevalent on the Mac, and even the safest browsing habits don't protect you completely. So, we now officially recommend Sophos as our Mac antivirus of choice.
by ALAN HENRY
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