Black Mesa is a scene for scene remake of the original Half-Life in the Source Engine. If that wasn't enough, the entire game was re-voice acted, re-modeled, and re-coded in order to provide the most up-to-date version of Half-Life possible.

You can see a whole bunch of screenshots over at the Source SDK community on Steam and you can download the mod for Source: here.

For a comparison of the two, check these out:

Half-Life (PC) image
Original Half-Life Rendering
Half-Life (PC) image
Black Mesa Rendering

According to the black mesa wiki, "This total conversion will not require Half-Life: Source to play - only a copy of any Source Engine game installed on Steam." The cheapest of these is 4.99 and can be purchased on Steam: here.