Here's a little tut on making fiery text using The GIMP.

-Start with your background and text of choice:
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-Duplicate the text layer. Select the bottom text layer and do Layer -> Layer to Image Size.

-Follow Albino's flame tutorial and smudge the top of the lower text layer to look like flames:
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-With the lower text layer active, do Filters -> Colors -> Colorify..., choose any flame-ish colour you wish:
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-On the top layer, select the text using the Colour Selection. Select the lower text layer.

-Invert the selection, then deselect the space outside the text using the Contiguous Region Select:
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-Deselect anything above the text:
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-Do Edit -> Clear. This will get rid of any flames that fill in spaces within letters:
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-Reselect the top text layer using Color Select.

-Do Filters -> Render -> Clouds -> Solid Noise... Set the X Size to 16, and Y Size to 2, but you might need to tweak it to make it look like the clouds are more stretched vertically than horizontally:
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-On the top text layer, do Filters -> Colors -> Colorify... and try to match your text to the flames:
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-Using the Blur tool, blur along the border between the flames and text to remove any hard edges:
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-Flatten your image, then do Tools -> Color Tools -> Brightness/Contrast. Set the brighness and contrast until your flames look hot and glowing:
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And there you go, some nice flaming text.