Fontify changes the game fonts to one of your choosing. That's it.
Extract Fontify.bat into "<Your WoW Folder>/Fonts/". If the directory doesn't exist, create it.
Drag a .ttf font file onto the script, as shown in the image.
(Re)Start the game.
How is it different?
It's not an addon, but a Windows batch script. You run it once with a font of your choice and restart the game. As a result, no extra memory or CPU cycles are consumed in-game.
How does it work?
WoW looks in the Fonts folder for four font files and uses them as the defaults if it finds them. Instead of installing an addon to change the fonts after the game is loaded, we can instead override the defaults by providing the fonts, under their particular names, in the Fonts folder. What this script provides is a quick way to do just that -- instead of manually copying and renaming a font file, you can simply drag it onto the script file.
What if I only want to replace certain fonts?
Open up the file in your text editor of choice and comment out the fonts you don't want to replace. For example, I prefer to use a different font for morpheus, so I prefix the line which copies morpheus with "::".