If you are going to pay, I recommend Sophos. Disclosure-we do resell them. We starting using them in the enterprise space where they are a big player as they are displacing Norton/McAfee. In medium to large installations, I find their management tools, end-user performance impacts, threat update speed, and prevention rate to be superior to the other SMB options.
If you are looking for free, I'm sure some folks will weigh in for that perspective.
