Half-Life 2 is currently being recreated within the original Half-Life by a team of modders, you can follow their progress on ModDB.
The reason for the de-make is “Why? Mainly, just for fun, but also because a lot of people like how the goldsrc engine feels, or enjoy the gameplay or Half-Life more than Half-Life 2. It’s also a way to see what Half-Life 2 could have looked like in the limits of Goldsrc, if valve never developed the Source engine.”.
The mod already has a demo available on ModDB.
The mod team are currently asking for feedback regarding art style. “We were trying to decide if we should stick to hl1 or hl2 style graphics. We still can’t decide, so we’ve made a poll so we can gauge what most people want, and we’ll try to adhere to the most popular option. You can take the poll here. Please be honest, as we’ll be basing most of the mod on this info.”