Tons of hobby operating systems exist, but hobbyists have built browsers too.
*Emacs has eww
*Webkit and Chrome were both born from KHTML, a FOSS project
*Servo started as a small research project
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It's really satisfying to use a profiler for the first time on a project. I always find a big performance win with only a small code change.
It's never the code that I expected to be slow, however!
Blogging about my programming language project: choosing the basic syntax!
https://www.garden-lang.org/blog/syntax.html
For hobby projects, I really like software where I can do small features or tweaks. Sometimes I don't have time for anything more substantial.
Website projects are great for this. Are there other areas?