Zen GM

Zen GM games are 100% client-side HTML5/JavaScript apps. This means that all the simulation is done on your computer and all the data is stored on your computer. There is no server-side component. This magic is brought to you by IndexedDB. There are some disadvantages, such as less than ideal cross-browser support and the inability to easily copy saved games across devices. However, there are some huge advantages, most importantly that my server costs are basically $0, so you can play as much as you want for free.