The GIMP comes with a flame plug-in with an interactive editor. The old documentation page is here.

MetaCreations incorporated the flame code into Kai's Power Tools (KPT) version 5 and dubbed it FraxFlame. It was updated in KPT6 and eventually sold to Corel. It's now available as the KPT Collection.

For animation, try the After Effects plug-in from neosapien, or vlam the Digital Fusion plug-in (download it here). A new version of AEFlame for OSX and AE5.5 is here as is the source code, thanks to Darrin Cardani.