Sophos free or paid is good, BUT interacts HORRIBLY with Apple Mail, so if that is in use, headaches and aggravation will be unavoidable, and another product is probably a better choice (many Mac users who do use Apple Mail are not quick to give it up). That said, the best (most worthwhile and capable of protection from current real-world threats to Mac OS X) products are the paid version of Malwarebytes, and BitDefender.
Same with the *free* BitDefender offering via the Apple AppStore channel. After the demo expires it's a passive (meaning when enacted by the user) check only. The full-fledged Malwarebytes with active detection/prevention is NOT free. Worthwhile products for the Mac that are free are Sophos and Avast (if you wish to use their product).
