To those who are unhappy with the graphics of HL1, you can download Black Mesa which is an remake of the original.
Mind, though, that development appears to have ceased on Black Mesa; and while it's basically 99% complete, I encountered a few show-stopping crashes and performance blackouts requiring some intense googling to bypass. I'd still recommend it, though.
It is a mistake to just play it and think you have had the HL1 experience.
For me as someone that played (and modded) the original when it first came out, it’s Jackson’s Hobbit movies vs the book when your first of many reading was years before.