Should I use antivirus?Yes. There are great programs out there that are lightweight and free, so your only investment is the time it takes to install. You're not only protecting yourself; there's a concept in medical immunization called herd immunity that's also applicable here. The more people on the internet who are running programs to protect their computers, the less likely it is for any of us to get infected. You're protecting everyone you email, IM with, or contact on Facebook as well.
Even on my Mac?Yes. Mac viruses are rare, but not non-existent, and all of the same reasoning applies. You have no reason not to do it, and every reason to do so. It used to be unheard-of for a virus to target Macs, but this had nothing to do with a special immunity of the operating system and everything to do with their market share. Mac adoption, especially in business environments, has been climbing upward, and that's made attacking them a viable business model.
Okay, so which antivirus should I use? What's the cost?If you're a home user, there's no reason you should ever pay for antivirus. For PCs, Microsoft's own Security Essentials is one of the best virus-fighting tools available, and is even free for very small businesses of up to ten users (beyond which Microsoft wants you to buy their Forefront product). Apple has yet to step up with their own offering, but Sophos is free for home users and is quite good.
Business users will end up paying for protection, but it shouldn't be terribly expensive. ESET's NOD32 product is one of the best in its class, and is very inexpensive as well as cross-platform. If you have an IT provider, go with whatever they're comfortable with and recommend; the differences between the product matter less than understanding the ins and outs of the interface and being able to manage it effectively.
What about Linux or UNIX, smart guy, do I need AV for that?Almost everything that I said about Macs applies here, but double because you were so smug about it just now. You're probably--probably--a little more security savvy than the average Windows or Mac user, so your machine is probably locked down a little better. However, you are by no means immune, and while Linux viruses are rare, antivirus is, again, free and relatively lightweight these days. ClamAV is popular, easily available, and works well.