Since viruses for Macs are, for all intents and purposes, still nonexistent, anti virus programs are really not needed.
In the event that an active virus does rear its ugly head,
you would likely get wind of it via the internet well before it attacks your Mac, and in plenty of time to take defensive action.
I do keep ClamX(it's free...well, it's donationware, and highly regarded):
http://macupdate.com/info.php/id/15850 anti-virus on my Mac and run it occasionally, but its never found anything.
Regarding routine maintenance, I also use Macaroni and recommend it highly. It automatically performs five important maintenance functions: Cleans daily Unix files,
Weekly Unix files, Monthly Unix files, Repairs permissions, and Removes localized files. You just schedule how often you want those functions performed and Macaroni does it automatically, reliably and dependably.
I also use Cache Out X:
http://macupdate.com/info.php/id/9538. I run it from time to time and definitely find that it helps keep my OS running smoothly and quickly.