Video CoPilot has a free fire project (CS3+) for download:
"Here is a little test I made using CC Particle World and some vector blur to make Fire. It’s not perfect but an interesting study. One of the key things I did was used a wiggle expression on the particle birthrate to give a variable emission pattern like something was burning inconsistently. It’s a little slow to render but it is very high resolution."
Update: Mathew Kelly shows you how to build your own with built-in filters:
After effects fire tutorial from Mathew Kelly on Vimeo.