Viruses / Malware
Malware / Spyware is basically non-existent on Mac OS X. Although, as with anything, it's never advisable to be totally complacent either i.e. sensible precautions like not opening attachments on spam is always a good idea.
You can get Mac virus scanners, but the only thing that they've ever caught for me is attachments which would only infect windows systems.
There were some issues with macros viruses which could impact on Microsoft Office for Mac too, but again, nothing really system wide.
In general, viruses for OS X have so far only been 'proof of concept' in labs. It's significantly more secure than windows as it's a UNIX-based OS.
Windows was never really designed from bottom up with networking in mind, rather networking was bolted-on to a system that was designed for use on a single PC, or a closed office network.
That's where current generations of windows tend to have their downfall. There are lots of exploitable holes in the security systems.
Mac OS X, and other UNIX systems however were built to network, that's what they do. UNIX started it's life as an operating system for giant mainframe computers which had to share resources with absolute security. Processes are kept separate, memory is protected, etc etc In fact, UNIX systems still dominate the entire internet infrastructure, communications systems, telephone networks etc etc.
Rather than being a personal computer operating system adapted to network, UNIX is a network operating system that was scaled-down to operate on personal computers as Mac OS X, various Linux systems, Sun Solaris etc
Because of this, they tend to be much more rock solid platforms.
In general, if you're running Mac OS X or Linux as your primary operating system, viruses / malware are generally a non-issue almost 100% of the time. While they are technically possible, there are almost no known viruses for OS X at the moment versus tens of thousands of them for Windows.
It's one of the huge advantages of not running windows, but Apple and Linux distros generally don't like to shout about it as they don't want to encourage virus-writers by giving them a challenge.