Siri seems to get new powers all the time, but it's still not useful unless you're really hooked into Apple's apps. If you want to add some new commands and don't mind a slow response time, GoogolPlex is a clever little hack that let you control Spotify, a Nest Thermostat, and more.
GoogolPlex uses a man in the middle style attack to grab the commands you send to Siri and send them to GoogolPlex instead. Because of that, it's a little sluggish since it routes from Siri, through their servers, and over to Safari. You'll only need to change one setting on your iPhone and create an account to check it out. Once you're set up, you can use Siri to control a ton of different things. Considering you're basically opening yourself up to attack, it's probably a good idea to disable it when you're not going to use it. Still, it's a clever workaround that adds some much needed third-party functionality to Siri.