Just a few more quick recommendations other than the links I already provided:
Perian
Flip4Mac
VLC
Handbrake
Plex
Carbon Copy Cloner
Adium
Skype
While you do not need AV for your Mac, there are a couple of things to consider:
1. There are trojans out there, but they require you to input your Administrator's password. The best defense is to use good common sense. Know what you are downloading and installing and only do so from reputable sources. There are no known viruses, but nothing prevents you from installing malicious software if you willingly give permission.
2. Your Mac is immune to Windows viruses, but it can be a carrier. You can pass on a virus to a Windows user via email or shared files. That is where ClamXav can be useful in scanning your files (you can set it up to be selective about what it scans I believe). If you share a lot of files with Windows users, they may appreciate you practicing safe computing and not spreading their viruses.
3. The best thing you can do to ensure your computer is safe is to have a good backup strategy. I use both Time Machine and Carbon Copy Cloner. They both have their strengths. Time Machine allows you to recover files from the past, but it is not bootable. A clone allows you to boot immediately should you experience trouble with your primary drive. Both can be scheduled for regular backups.