security programs mountain lion

No viruses affect osx at this time. According to Apple - http://www.apple.com/why-mac/faq/#viruses

Is a Mac safe from PC viruses?

Yes. The OS X operating system isn’t susceptible to the thousands of viruses plaguing Windows-based computers. And although no computer connected to the Internet is completely immune to all viruses and spyware, OS X has built-in defenses designed with your safety in mind. The Mac web browser, Safari, alerts you whenever you’re downloading an application — even if it’s disguised as a picture or movie file. And Apple continually makes free security updates available for Mac owners. You can even have them download automatically.

As for malware - Free Software:

ClamXav

iAntiVirus

FlashBack Check

no script

noscript.net/

ghostery

www.ghostery.com/

You dont need antivirus, however most here have ClamXaV

https://itunes.apple.com/us/app/clamxav/id430207028?mt=12

The majority of Mac "pay for" security programs will hurt performance, and some cause problems worse than those they claim to solve.

For a more complete answer to your question, see my Mac Malware Guide. It will tell you what the threats are and what you need to do to keep yourself safe.

Just a few comments about some aspects of other replies... I would strongly recommend against iAntivirus. Not only do Norton/iAntivirus not do a very good job of detecting Mac malware, but iAntivirus has no facility for updating its definitions. It only updates its list of known malware whenever there is an application update, which is fairly infrequent.

Flashback Check's site no longer exists, apparently, and for good reason: Flashback is extinct. You won't see it anywhere at this point other than on very, very outdated computers that have been infected since last year or in a malware hunter's collection. Thus, there is no need for a tool specifically designed to protect against Flashback.

NoScript and Ghostery do not protect against malware, strictly speaking. They have absolutely zero power to detect malware of any kind. Properly used, they may prevent certain kinds of attacks that can occur through things like malicious ads or cross-site scripting. However, such things, by themselves, are not sufficient to infect your machine with malware. They only have the power to annoy you or to try to convince you to hand over personal information.