Changing Fonts

You can use a mod to change in game fonts. Or you can replace the original game fonts with the fonts you want to use. Create the following folder:
<WoW directory>\Fonts\
Create copies of the font file of your choice, place them in the above folder, and rename them to the following names:
FRIZQT__.ttf (the main UI font - NPC names, player names, spell names, item names, buffs, quest log text, combat text, button text)
ARIALN.ttf (chat font, info text, small text)
skurri.ttf (default unit frame combat text)
MORPHEUS.ttf (mail text, quest log header text)
FRIENDS.ttf (friends list text)

Popular Fonts: Expressway Free, Comfortaa, SD Fresh Regular, Tork, Andika Basic, SF New Republic
dafont has many good fonts to choose from.

Font Mods

Ansi Combat Font Ansi

This AddOn changes the Damage and the <Floating Combat Text> floating behind your character. 
WoWInterface, Curse

Better Font grdn

Little tweak which enables a new default font to your UI. Only for styling purposes. Wastes almost no memory.

SharedMedia Elkano, Seerah

Inserts media(fonts, bars, borders, etc) into LibSharedMedia's storage. They can then be selected by any addon that supports LibSharedMedia, a lot of them do.