This is a personal test I did to see if I can achieve a flickering light. I have used the internal renderer but this is compatible with Cycles too with a few modifications. You can also use this in a game because it uses internal renderer.
This works by animating the 'energy' property of the lightbulb and using the noise modifier on it. The settings can be altered to fine-tune the flickering to your liking.

P.S. Please ignore the description in the .blend file. I was essentially 10 years old when I created this.