Which of your must-have free software choices cannot be used on a Mac?
are you referring to a specific list? We have had Must-Have lists for both Mac and the PC.
I will assume you are referring to this article:
Roundup: 15 Must-Have Free Software Programs for Your PC
Indeed, some of these programs are also available for the Mac. They are:
1. Firefox / Chrome
7. Google Picasa
13. VLC Media Player
For the others there are certainly alternatives.
Check out this article for a Must-Have Mac list:
15 Must-Have Free Apps For Your Mac & Giveaway