Over at Halflife2.net you can find two screenshots which show you what the Source engine port of Half-Life 1 will look like. The game hasn't been totally overhauled like Counter-Strike: Source, but HL1 on the Source Engine will still benefit from shader graphics, the physics and other new engine features.
I think it will be free with Half-Life 2. Or perhaps to all half-life (1) owner via steam?..