Comes out next year! It's a remake of the original 1997 Game of the Year, Half-Life, only completely redone in source (not ported like HL:Source) for excellence of graphics. It will only require a Source game, and if it's like other mods will probably be free.
A new trailer was released recently. I am thoroughly impressed.
youtube
youtube HD
downloadable here, highest quality
Official Website
edit: There was a name change for the project I was not aware of. ":Source" was removed by the developers due to this. As a HL2 mod it is not being produced by Valve and will be free of charge according to their FAQ here.