#3 On my list of "Top 10 Mac Applications" is ClamXav.
One of the reasons you bought your Mac was most likely the thought of "No Viruses". And good for you because it's true, Mac hardly has any viruses for it.
But even though there's not many doesn't mean you can't get them.
Your computer has all your pictures, music, videos, documents, files... why take the chance losing that all?
Even if it's a low chance, it's a chance so be cautious. This is why I will recommend ClamXav.
It's a free Virus Scanner that works just as well as the few other virus scanners you can buy.
You don't need to scan everyday or every week (not that you would want to, ClamXav is quite slow) but every 1 or 2 months would be nice.
This way you can always be sure your stuff is safe and nothing has found it's way on your computer.
Definitely try it out, it's not a big thing to have and it may save you a lot of stress in the future. Here's the website.
ClamXav Website: http://www.clamxav.com/