none. unless you do something very silly you are very unlikely to get infected. linux/unix doesn't assume you want to run everything like windows does.
linux is good when it comes to viruses.. so u shouldn't need an anti-virus..
panda is a very good windows antivirus.. so you can get the one for linux, they are offering it for free.. http://www.pandasoftware.com/download/linux/linux.asp
although there are many others .. i have never heard of any of them since i dont use linux.. so you might have to try reading through them..