Aside from iLife '09, iWork '09 and M$ Office 2008, I would say that the must have software on my Mac would be: Handbrake - for video encoding/converting. VLC Media player - for video playback. VirtualBox - it's like VMware Fusion but is available for free beta. hulu Desktop - Recently got rid of my cable and use hulu for TV Boxee - Another reason to kill your TV Transmission - a nice lightweight torrent client. PS3 Media Server - Stream files from your mac to your PS3 over network Gimp - Great photo editor software Braid - One of the best games out there. Everything on my list above is free. Except for Braid ($15).
