Well, the traditional method is to use a normal old fire sprite. sprites/fire.spr is a good example.

The trouble with this is that the sprite will always face the player - by default sprites are NOT axis locked.

The first response to this was to create an animating texture out of the sprite and place it on brushes. This looks pretty good, but it can hurt the framerate a bit if used excessively.

More recently, however, some mappers discovered that if you hex-edit the sprite, you can make it axis locked either on the up-down axis (so it won't "tilt" when you are above the fire), or completely locked in place (it acts like a polygon facing the direction specified in the "angles" key). There is a tutorial on hex-editing sprites here:

http://collective.valve-erc.com/index.php?doc=1063191790-06027400

Of note is that there is a bug in half-life so if you make the sprite completely locked, none of the angles can be 0 or you will get weird results. To fix just make any angle you want to be 0 into 0.1.

Some people have written programs to do this hex editing for you, spritemage (german):

http://www.slackiller.com/hlprograms/spritemage.zip

spritefixer (english):

http://www.slackiller.com/hlprograms/spritefixer.zip

The best method for fire is usually to make your fire sprite z-axis locked (faces the palyer horizontally but not vertically).

There are many sprites out there both axis locked and not, one of my personal favorites is the following one (origionally by meat, I have converted it to a spr and axis locked it):

One thing about the spritefixer tho (I've heard this, but haven't seen it, personally) is that in Overview mode, the sprite takes over the whole map.

A guy who makes models for me sometimes said that. In the map, the sprite was scaled to 1. But he said that in the overview, the whole map was covered by the one sprite...and this sprite had been made with the spritefixer program.

I'll do a little more checking into this and get back to you about it.