Some time ago, a friend got me into an online card game called Urban Rivals. It is similar to other card games such as Magic the Gathering, but played online either via iPhone/iPad app or in your browser. One aspect of the game that quickly got my attention (other than the game itself), was the card marketplace where you can buy and sell playing cards.
It didn’t take long for me to realize that it was an inefficient market. Profitable trades presented themselves quite regularly, and with a bit of automation it would be easy to take advantage of the market. I proceeded with developing a Google Chrome extension that would allow me to do so, and it worked quite well.