That is a great question, Franko...
Some guess by me:
1. Atari's Unix
2. Amix
3. CP/M - as mentioned about but that actually had a lot of software and I think it was free up to a certain point. I know when it came to the IBM PC it was like $1k compared to MS DOS so it didn't sell well then.
4. Atari 2600 basic :-)
Cheers!
-P
