This does not mean that you should be completely careless on the internet, but simply that there are no "in the wild" viruses.

The main push for mac antivirus has to do with passing along viruses. Because mac’s cannot get windows viruses, they can open files that are infected and pass it on to a windows user without even knowing it. Many people believe that for this reason, every computer should have an antivirus, so viruses can be stopped before they are passed on. Personally, I do not use an anti-virus on my mac, but I am cautious in my online ways.

EDIT: As for what sandy is saying about registry cleaners, Mac OSX do not *have* a registry… Some responses on here seriously scare me.

The only malware that is normally found on macs, were pretty much found only in pirated software and porn sites.

Some reasons as to why a mac uses may still want an antivirus for a mac is because to prevent viruses being passed to a windows system, a required business security policy, or they go to pirate and/or porn sites.

Spyware is designed to intercept or take control over the interaction you have with your computer without your consent. Spyware collects personal information (in addition to your passwords and surfing patterns, good spyware can also get credit card and bank information), redirects your browser (I inadvertently downloaded spyware one time that would send me to the same advertisement no matter what web link I typed in or clicked on), and spyware can actually change your computer settings affecting how you connect and how fast you connect to the internet. Anti-spyware protects your computer from spyware. A good anti-spyware program will catch spyware before it infests your system and a great program will catch in and get rid of it once it’s on your computer. It is important when deciding to use anti-virus software to make sure you are at a legitimate website before downloading.
Viruses (also called malware) are similar to spyware except they are not designed to find personal information. Viruses are designed for the purpose of destroying your computer. They do this by deleting and partially deleting files throughout your computer. Viruses are most easily spread through email and websites. However, viruses are also attached to .exe files. In order for the virus to work, it must be executed. These are just as damaging to your computer as spyware. Strong anti-virus software is a must if you are connected to the internet and even if you’re not as viruses can piggy back in through burned software or floppy disks.
Registry cleaners are another thing you should be aware of if you’re a computer user. The verdict is out on this one. Some experts suggest that because Registry Cleaner removes data that is no longer in use (these are designed for Windows OS) it frees up space and helps eliminate data that could be slowing your computer down and since manually cleaning out your computer register would take a ridiculous amount of time, these programs are beneficial. On the other side of the argument, experts argue that using a register cleaner makes a user more open to malware infection, and does not affect the speed, especially if there are multiple users with their own log ins. The jury is out on this one.

McAfee Antivirus is probably the best……….

I think as long as you don’t go hogwild on the internet, and downloading random stuff, you’ll probably be fine. Just be aware in the near future things may change and you may want to run an AV.

I used the below website to give you an idea of what is out there for your Mac. Im sure the version numbers on that site are out dated, but the same people still obviously make the software.