From now until June 24, you can get the WoW Battle Chest (including the original game, The Burning Crusade, and Wrath of the Lich King) for only $5 US. 2010's Cataclysm is also $5, and the recently launched Mists of Pandaria is half price at $19.99. That means you can get the complete game and everything launched since 2004 for only $29.99.
Head over to the official summer sale page for more info.
[Thanks to Sharvis for the tip!]