You might have seen this in Ben Dansie's Durian application demo reel (after being selected as one of the final Durian team members), but he has recently come up with a very cool way to make flames and fire using Blender.
Without volumetrics, raytracing, even particles...
Can be changed to a fair few shapes, but can overall only go in one general direction because the movement is controlled by procedural textures being dragged by empties.
The setup can be tweaked simply by changing the speed that the empties move and the scale of the textures.
Theres the thread on Blenderartists.org here. And also links to 3 different versions of the fire below (click the images to view on Vimeo).
Ben has also very nicely put the .blend up for download, so we can all learn this technique and start making our own cool fire! Download from this link or from the thread on BA mentioned above.
Bat Symbol fire: