Are there any websites with lists of self-guided themed tours?
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Accepted answer
These aren't crowdsourced sites, but there are some excellent professional ones. A few examples (I'm making this a community wiki so others can add their own):
Rick Steves Audio Tours (Europe) - free, available in an app that supports downloading and maps. Most are generalist walks around city centers, but some are specialized, particularly the museum tours
Detour - collection of well-produced audio tours for some US and world cities, most around $5/each, including a number on specialized topics (e.g. history of the Black Panthers, geology and earthquakes, spies, etc...)