Black Mesa (Source)
If you’ve played the Half-Life storyline, Black Mesa (previously named Black Mesa: Source) is the perfect way to relive your memories. Everything up to entering the Xen portal is completely remade and improved via the Source engine. It will give you enough hours of gameplay, so take your time and enjoy it. If you think it will be easy, I guarantee you won’t beat the game without dying several (many) times on Hard difficulty.
The game is free. You can check it’s page and download it from The Black Mesa site. If you loved the original, you’ll love this too.