There's 9 Half-Life 2 trailers available to download from Gamespot (need to be a member) and there's also a free 11.9 MB movie teaser in the QuickTime format at IGN.

Thanks to Philibob! I'll add more mirrors as I find them.

IGN

Half-Life 2 Teaser 11.9 MB

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Gamespot

9 Half-Life 2 trailers @ Gamespot

9 GS Half-Life 2 trailers free 31MB halfway down the page

9 GS Half-Life 2 trailers free 31MB