Right now you can get Intego's Virus Barrier (in my opinion the best anti-virus software for the Macintosh), bundled with Net Barrier (two-way firewall and anti-spyware) for only $20 for one year (usually $50). You must use this link to get the deal:

Intego - Mac Internet Security X9

I don't know how long this deal is going to last.

Before anyone asks....no, I don't think that you *need* anti-virus software for your Mac. But there are a number of users who want it anyway, for one reason or another.