In 2017 I devoted some time to learning about blockchain and cryptocurrency. I read the original Bitcoin whitepaper and source code, and all manner of other related literature.
That summer I started a company with a friend and set about building software to scan for arbitrage opportunities across cryptocurrency exchanges. The software allowed for different exchanges to be queried from a single API so that their spot prices and order books could be compared easily. Parallel HTTP requests were supported for performance and throttling was included to respect API rate limits.
Python
Python Requests, gevent
Vim
The arbitrage scanner featured a text based interface. A source code sample is more interesting.