http://cg.tutsplus.com/tutorials/blender/introduction-to-smoke-simulation-in-blender-2-5-day-2/

In the second part of our invaluable, in-depth series looking at smoke and fire simulation techniques within Blender 2.5, Gottfried Hofmann shows us how to create fire! The incredible (and let’s not forget, completely free) result could be used as a flamethrower, dragon breath, a rocket trail…the possibilities are endless!