Please be sure to search the forums as this is a very common question (as in weekly). Look up info on ClamXAV and iAntivirus. Also, as you'll often hear, Mac OSX has no true in-the-wild viruses, only a couple trojans that are kind of hard to get.
Huh? To the best of my knowledge, antivirus software comes in two flavors--scanners and disinfection tools. Most do both. I have difficulty even conceiving of a utility that could prevent a virus without scanning for infections. If you have specific examples of some that do, then by all means share their names.
Before you do that, make sure that you understand the meaning of prevention. MacOS X's malware prevention is an integral part of the OS design, not a third-party commercial or freeware add-on. These include the UNIX permissions model and the MacOS X's object-oriented design. Some of Apple's controversial business practices also prevent the distribution of malware. An example of this is the fact only Apple DRM is allowed on iTunes and the iPod. Malware distributors cannot buy a DRM license and then use it to distribute malware on the Mac.