Yes, Black Mesa costs money but it’s absolutely worth it, they put a LOT of effort into restoring the original as faithful as possible but with more modern graphics (Source engine based). I played the original Half-Life when it came out and Black Mesa definitely gave me the same amount of awe and wonder. One of the best fan remakes of any game I’ve ever seen.

Also, I see that it’s currently on sale for only $5 (75% off), at that price it’s a steal and absolutely worth it.