ClamXav is a nice antivirus program that saves resources and runs on Mac OSX. After upgrading to version 2.8.1 it asks you to pay for it which is totally ok, since it is a good piece of software that has been available freely for 10 years now.
But if you are convinced with the next older version, you can still download it from the official site here:
PS: If you already installed the newer version, you should execute the engine remover script before installing.