Platform: PC, Mac and Android
AVG has been one of my most recommended anti-virus programs over the years. The free version does a really good job of protecting your machine. Unlike other popular anti-virus programs, it does not bring your system to a crawl. If you upgrade to the paid version, it will protect from even more threats. If you want full coverage including a firewall and spam protection, then get the Internet Security package.
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Free: AVG AntiVirus for Mac
Free: AVG AntiVirus for Android