Open Office, I actually beleived that till I convinced the company to buy it at one point...
Photoshop/InDesign/Distiller-Acrobat Can similar stuff be done on Linux? Yes. Do companies feel like spending millions retraining people to use it? No.
iTunes.
iPod uses iTunes to populate the iPod with songs. There are a copy third party utilities that can also do it but iTunes is free and usually far more uptodate/compatible with the latest iPods. I can't imagine not using iTunes with an iPod though my Russian freind does this.
I would guess less than 10% of computer users use photoshop, maybe 10% uses iTunes and MSoffice is very interchangable with OpenOffice for home users.. These are SWAG of course but nodoubt so were yours..