Avira is what we use (at a very security-conscious org), and it's been unobtrusive.
(reply to noondip): if anyones got a better suggestion I'd love to hear it :)
Back when Apple still made Mac OS X Server as a separate operating system, they included ClamAV¹ to scan for malware in mail. They don’t include it anymore, but ClamXav² (been around since 2004³) is a nice GUI for ClamAV that I’ve been using for a while now.
I run a private mail server and swear by ClamAV to help reduce noise and pollution that accumulates and spreads through my server, but I don't think I've ever had any luck with it being a good front line defense against up-and-coming malware, whether it targets Windows or Mac. I don't think I would recommend it as a primary malware scanner for a Mac, or Windows.