What's the best anti-virus program for my iMac?

ClamXav 2.2.5. But if you are concerned about the recent malware issues then see:

Helpful Links Regarding Flashback Trojan

Visit Thomas Reed's site for insight and help: Mac Malware Guide

A Google search can reveal a variety of alternatives on how the remove the trojan should your computer get infected. This can get you started. However, be careful about what you do as new variants of the malware circumvent the efforts of earlier tools.

Also see Apple's article About Flashback malware.

Apple has released Java updates for Snow Leopard and Lion users:

Java for OS X Lion 2012-003; available only for users of Lion with Java installed.

Java for Mac OS X 10.6 Update 8; available only for users of Snow Leopard.

Flashback malware removal tool; available only for users of Lion without Java installed.

Install whichever shows up in Software Update. It removes the malware (if present), updates Java (if present) and tightens up Java settings for the future. You may download from Apple's web site instead of using Software Update, but it's important to know which one to get, because the other two won't work for you.

For the truly paranoid see 10 Simple Tips for Boosting The Security Of Your Mac.

You don't need any.

Most, or possibly all, antivirus software is malware on a Mac. The key is to be websmart and don't open any stupid emails or download attachments that you don't know. It is pretty simple really. Instead of spending time looking for antivirus software you should be looking at how to make sure you don't get one of the three viruses that have infected Macs in recent weeks, all of which have been fixed with software updates.

Is there a removal tool for Snow Leopard users?

Harden your Mac against malware attacks

Is there a removal tool for Snow Leopard users?

Yes, run Software Update.

Totally agree with stevejobsfan and silly rabbit. You really don't need any antivirus. Keep away from the trojans like "Mac Keeper", that are unethically advertised everywhere in the world wide web. Take emails from unknown people, applications from unknown publishers with a pinch of salt.