Escape from City 17  

Here's a great fan-made short movie mixing live-action with some in-game assets (mostly enemy models, sound effects, and some visual effects) from HL2.

Very impressive work considering the budget was apparently "less than $500". It was put together by the Purchase Brothers, a small film production company that's done mostly some commercial work until now. Apparently the City 17 shorts were originally created as a test-bed but have evolved into a series, so expect more to follow.

Seeing the HL2 world in live-action shows you how well the fiction stands up to adaptation. Even the 'uniforms' of the resistance members end up looking a lot less dorky than I would have expected.

In case the link above doesn't work, you can check out a lower-res version on YouTube. With almost 1M hits (as of this writing), the piece seems to have brought the Brothers some much deserved attention.

Hopefully Valve is watching and gets these shorts up on Steam.