Google Now offers a wide range of voice commands designed to work with your natural language, but it can help to know exactly what the virtual assistant is capable of understanding. That's where OK Google comes in -- it's a growing, searchable list of every known Google Now command.
There are plenty of lists out there and none of them are truly complete, as Google is constantly adding more functionality to their service. Thankfully, OK Google will eventually be open source so that anyone can contribute to the list. Right now it stands at about 150 commands with over a thousand variations, organised in a clean interface that also highlights the variables.
Developer Kitze has just launched the site, so it's no doubt going to grow in functionality as it gets more use and feedback. Check it out and learn some new ways to put your virtual assistant to use.