Operation Black Mesa is a complete recreation of Half-Life: Opposing Force using the Source Engine.
I joined the Operation Black Mesa team in 2007 as a Level Designer, I mostly worked on the chapter Crush Depth. The same year, we finished in second place for the Moddb’s Most Anticipated Game of the Year.
Unfortunately, I had to leave the team a year after due to lack of time.
More information can be found about Operation Black Mesa on their Moddb’s page.