Calcite is a neat project that uses existing projects to suggest code snippets. https://edelstein.pebbles.cs.cmu.edu/calcite/ (hasn't hit the industry AFAIK)
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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?
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!
Over a sufficiently long time horizon, all code you write is legacy code.