Following the news of Team Fortress 2's free-to-play plans, Blizzard is introducing a new, extended trial period into World of Warcraft. Previously, newcomers could play the game for a limited amount of time before they either had to start paying or get out. But now, anyone can jump in and play any number of characters for as long as they want for free; the catch is that you're capped at level 20.
If that inspires you to enter the full game, you'll be able to then purchase it and the Burning Crusade expansion for $20. Anyone who currently owns the standard version of World of Warcraft is automatically getting The Burning Crusade for free.