In the map Metro, there is a really nice fire effect made up of several different textures, which depict different frames of the flames, all set upon a black background.
How do i make the fire?
Rub a couple sticks together.
lol
That thread about 'Aliens invade a school' has links to the map. There is some VERY impressive fire outside (plus a really awesome alien space ship). I'd recommend downloading that map and looking at the shaders.
^^^^ Topic in question:
http://tremulous.net/forum/index.php?topic=10973.0;topicseen
just do what DeathSkull said XD