Software code is considered open source if it meets OSI's Open Source Definition - not just making source code available for others to look at, but licensing it in a way that allows others to redistribute, modify, verify, and in some cases even resell it.
Different people value open source for different reasons. For some, it's a learning tool, as you can directly read the source code of many popular and well-architected applications. For some, it is a way to ensure that the software they use isn't spying on them. For many, it's a way to maintain control of your computer, knowing that you are free to modify any of the programs you run and suit them to your needs.
For me, open source is about transcending ego. Once the work is out there, it belongs to the people, and they can do what they want with it. It's a statement that you want this software to live on, even when you're not around. That you're okay with anyone else creating and releasing their own spin on it, even if it's a completely different vision than what you had in mind.
Of course, maintaining a successful open source project is hard, and it's often thankless volunteer work. Trying to create a business plan around it is even more difficult, because nothing stops another team from essentially "stealing" your product. However, a business plan is often necessary in order to incentivize the design and engineering labor that goes into a polished product.
This is why, while I use open source software whenever possible, it is sometimes a tradeoff between idealism and polish. Much of what I use is proprietary (closed source) because in our current system, profit-driven teams are often better at producing consistent quality work (anyone who has tried to use Linux for music production is viscerally aware of this). That said, there are a lot of great open source projects out there that I do use, funded either through donations or through clever business models where the code isn't necessarily the product (e.g. enterprise support plans, advanced online features).
- Linux (I'm running it on several computers at home, one of which is a Steam Deck. If I didn't need macOS for music software, I would be solely running Linux)
- VLC media player
- Firefox
- Audacity
- Blender
- MediaWiki (the CMS that powers Wikipedia)
- Pure Data (a patching environment for audio signals)
- Zed (a speedy VSCode alternative written in Rust)
- MusicBrainz (a music metadata database that powers many websites including LastFM)