Hi mitschd, AVG Anti Virus, Avira AntiVir and Clamwin are all for free. The latter doesn't have a realtime function, though.
The big three are usually recommended - AVG, Avira and Avast. There are also some other quality contenders such as Panda's Cloud AV and Microsoft's MSE both of which are about to be released from beta. In the end the av that performs best for your needs and on your computer will be the one for you. All the above are free to try so you may want to try a few and see which work best for you. And if you search here on Wilders you will find plenty of posts about the above av's as well as a few others.