There's also free ones available in the MAS.
Aren't most of those on-demand scanners only? iirc the Mac App Store won't accept anything that installs a kernel extension, which is required for real-time scanning. Not that on-demand scanners are bad, but for a more casual user I think a real-time scanner is much easier to use.
Appletise said:
l have always heard that it is unnecessary to have anti virus for macs, but not too sure about not sure about that if downloading torrents
With Macs it's kind of tricky; since Macs account for a relatively small percentage of desktops most viruses aren't geared towards infecting Macs. However, that doesn't mean there aren't any, as we found out with the fairly recent Flashback malware. While OS X is pretty robust security-wise, even a user who browses carefully isn't completely immune to threats thanks to other recent issues such as certificate and DNS spoofing or the Heartbleed threat, all potentially allowing legitimate, normally trustworthy sites to be compromised.
So yeah, short answer is that I prefer to have antivirus than to not have it; even with antivirus software you aren't totally protected, but combined with some care in what you download, what e-mail attachments you open etc. it helps to cover you against the majority of threats out there. When it comes to torrents it's also fairly unlikely you'll be exposed to anything an antivirus program can't detect, as they're not exactly the fastest attack vector. Like I say, if you're downloading established torrents, rather than ones that have only just gone up, then you can usually feedback on the torrent to decide if it's trustworthy or not.
Anyway, I'm currently using
Avast! Free Antivirus for Mac and I'm happy with it as it's unobtrusive and offers good protection; it also seems to better identify phishing sites than Safari's built in mechanism, and you can get it for Windows as well (convenient for me since I use Windows as well quite frequently).
As bbfc points out
Sophos for Mac is another popular free option. Personally I had trouble with a persistent false positive, but it seems to be a rare issue.
I'd recommend giving both a try; install one for a week or two, then uninstall that and try the other for a week or two, and decide for yourself which you prefer. But if you just want my own recommendation then for now it's Avast!