The way to do this is very simple, and yet isn't very well publicized.
First, go into your World of Warcraft folder, wherever that might be. This is the same folder that contains the Interface and WTF folders. Then create a folder called Fonts. What next?
Well, first, a word about the fonts Blizzard uses.
The main font that Blizzard uses in World of Warcraft is called Friz Quadrata. You find this on most of the default UI elements. The second font is Arial Narrow. This is the font that you'll find on things like the username and password fields on the login window. The third main font is Morpheus, which is the font that's used for quest titles in the quest log and for in game mail. Finally, theres Skurri, which is used for the damage text on the default unit frames and such.
To replace these fonts, you simply have to choose a font to replace them and copy those fonts to the World of Warcraft/Fonts directory. You will then have to rename them to replace the default fonts. Here are the file names you need:
- Friz Quadrata -- rename your replacement FRIZQT__.ttf
- Arial Narrow -- rename your replacement ARIALN.ttf
- Morpheus -- rename your replacement MORPHEUS.ttf
- Skurri -- rename your replacement skurri.TTF
There are lots of nice fonts out there. I use a couple of variations on the Myriad font set by Adobe. These are gorgeous fonts, but not freely available. You can buy them from Adobe if you like them. There are lots of free fonts available all over the web, and I'd recommend trying a few out in game to see how they can change the look and feel of your UI.
Some particular recommendations now. For replacement of Friz Quadrata as the main game font, if you run Windows Vista, you should have a font called Calibri available in your Windows/Fonts folder. I like the Calibri Bold font for this purpose. You can simply copy the font file (calibrib.ttf) to World of Warcraft/Fonts and rename it FRIZQT__.ttf. I like the freely available Black Chancery font for Morpheus replacement. Arial Narrow has been tough to find a good free replacement for, but any decent condensed font should do well. Just test a few out. For Skurri, I like another freely available font called Ever After. I also use this font for my scrolling combat text.
Like all of the UI modding stuff, it's fun to play around with these fonts once you know how to replace them, and I highly encourage tinkering around with them to see what you like.
And just a quick update on the UI status: I've got fully functional replacements for most addons now, although I'm still getting some library conflicts that are keeping me from posting 1.0. I will post it as soon as I have it.