My Mac comes preinstalled with...
* Androgeos Exeunt checks.
...Symantec AntiVirus for Mac, which is basically a renamed version of Norton AntiVirus. It automatically checks every storage device inserted into it for viruses. That's how I found so many Trojan horses on the thumbdrive I used to store near-critical data files. Scans are not done or scheduled and require me to do them manually.
There are other antivirus programs for the Mac; Wikipedia lists Avast! Professional, MacAfee AntiVirus, PC Tools' iAntiVirus, mine, Sophos Anti-Virus, BitDefender, Kaspersky and Clam AntiVirus as being Mac-compatible.