As far as what comes with Panther or Tiger, Safari, Mail, IChat for Aim, Iphoto & Itunes.
As far as non Apple made software, Textwrangler is a GREAT text editor, Adium for a Multi Chat client, Colloquy for IRC, and Photoshop.
There is more but those are some of my real favorites. I do like IWork and hope Apple adds a Spreadsheet soon. In the mean time, NeoOffice is very nice.
Hard to pick just one item!