The sixth (and hopefully final) preload of Half-Life 2 has been released over Steam. This time around, the preload contains the Half-Life 2 maps. You can download the preload by restarting Steam. You can read VALVe's official statement below.
The sixth phase of the Half-Life 2 preload begins to Steam account holders today. This will allow users to download the Half-Life 2 maps in encrypted form. No purchase is required to pre-load Half-Life 2, but is required to activate the game. The moment the game is made available, those who have pre-loaded and purchased the game via Steam will be ready to start playing.Just as a final word of warning, this is a very large download.