iAntiVirus is an application for Mac OS X Leopard that protects your computer from malware specific to this operating system. Its threats database covers viruses, trojans, adware, and spyware.
iAntiVirus uses a resident protection system called OnGuard, which stays always alert for possible threats. You can do three types of analysis with it: a quick one, a full scan, and a customized one (all of them analyze running processes).
iAntiVirus can be used for free as long as it's not used for commercial purposes.