Black Mesa source is a Half-Life 2 mod, to recreate the original Half-Life 1 using the source engine, only with updated graphics, level design, and model design, and as a result of those updates, an updated storyline as well.
Now, on topic, I wanted to say how I really appreciate total conversion mods, and how they extend the life of games. Some of my favorite mods for games like half-life are Sven Coop, which allows you to play Half-Life 1 single player, cooperatively as well as custom maps, I also enjoy BrainBread which is a zombie mod.
Many mods, such as Overdose, for Painkiller, get picked up by the original game publisher, and then turned into an actual retail product, which is another item I'm extremely pleased with. Game modifications help not only the gaming community with extra hours of gameplay with a game, but also with providing the developer with new employees and products :)
What are some of your favorite mods, and why? Also feel free to share videos to those mods as well.