The best software should be the bit between your ears. If you're sensible, have a basic firewall (say, Zonealarm), have your browser (and email software) configured not to run executable stuff without positive confirmation from you, and have your cookie settings set to match your personal level of paranoia then in my view you don't need to clog you machine up with highly intrusive stuff like Norton or MacAfee - they just generate lots of irritating pop-ups and consume relatively huge amounts of processing power for no real gain. To a large extent they build on the mindset du jour of manipulation of the people by fear.
Having said that, very occasionally shit happens, and then it's probably worth having Spybot S&D, CCleaner & AdAware ready on your machine.