A small list of some of the things one might create with open source software, taken from my personal experience and usage patterns. This list is focused on content creation and management.
1. Write a book. (Finishing up my 2nd book’s 1st draft, comprising a combined word count of over 100,000 English words)
2. Write a thesis or dissertation. LaTeX support makes scientific and mathematical notation easy.
3. Organize your photos. Tags and easy one touch editing makes managing your photograph collection easy.
4. Edit your photos. Gimp provides all the cropping, filtering, color correction, airbrushing, and photo”chopping” tools that you need. For serious, technical photographers that need to do thousands of repetitive edits there’s an incredibly powerful command-line tool called Image Magick.
5. Design an advertisement for your business.
6. Design a poster for an event.
7. Compose a symphony. Or a short jingle.
8. Make electronic (sample-based) music. With drums!
9. Record music.
10. Record and edit a podcast or oggcast.
11. Design a type-face or font.
12. Write a script or screen-play.
13. Create a 3D graphic film.
14. Publish a print-ready document.
15. Design a complex 3D model like a car or internal combustion engine.
17. Write or translate subtitles for a movie.
18. Edit a movie (Application: one two three four). If you’re lucky that is, this is admittedly one of open source’s weak links at the moment, but is improving every day.
19. Do interior design digitally before actually moving your furniture around.
These are just a few ideas. I’m sure there are countless other things people use open source software for every day.