I've never used it, but I know a lot of people who do. For years, AVG have offered a free anti-virus product. You have to put up with the fact that it will put a signature on the end of your email messages, letting the recipient know that AVG Free scanned the email for viruses.
They offered a "Pro" version that you had to pay for. It has extra features that the free version does not. It doesn't add a note to your incoming and outgoing email. Anecdotally, it may get virus signature updates faster.