I was playing a MMO with a market. Sometimes people would make mistakes, e.g. selling 100 iron for $1,000,000 instead of selling 1,000,000 iron for $100. I made a little tool that polls the API and sends a notification when someone made a mistake.
Eventually I got bored of grabbing these mistakes and left the tool with my clan. It had the side effect of getting people active on Discord, and making people more actively involved in countering raids. We ended up building one of the most elite clans in the game until some other whiz kid built a better bot.
There was also this little augmented browser tool that calculates the best order and timing to attack in the game, and the statistically likely result.
We'd joke that all the tools we built for that game were probably well worth hundreds of thousands of dollars and that's around the time I decided to quit and spend my time on something more useful.