Black Mesa is finaly out (well, 2 weeks ago) after YEARS of developement by some hardcore fans.
It's a Half-Life 1 remake with the engine of Half-Life 2.
The game is on greenlight and won't remain free for ever, I guess.

How to install the game:
1) Go in tool panel and install Source SDK Base 2007 - steam://run/218

2) Download installer and archive (don't extract) from this page - http://www.gamefront.com/black-mesa-is-live-download-it-here/

3) Install then restart steam, so the game will appear in gamelist.

4) Enjoy the best all time fps with new graphics!