I once designed an esolang for my CS students to have fun with during the last week of the semester. A few of them used it in really, really creative ways and also went on to make their own languages for fun.
The one I created for class supports supports concurrency, regular expressions, HTTP requests, and file I/O. The challenging part is that every line executes concurrently in round-robin style (one command per line). There are only 8 commands and each line can only store a single string value.
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