Half-Life is 100% off (FREE TO KEEP) on Steam until November 20th!
https://store.steampowered.com/app/70/HalfLife/
The game celebrates its 25th anniversary by updating the Steam version with new (restored) content (Half-Life: Uplink, new multiplayer models and maps, old Valve intro), optimizations for widescreen, and brings the main menu back to the original 1998 version.
Secret Tape made a documentary about the development of the game with the original developers reunited, which I'd recommend you to watch: https://www.youtube.com/watch?v=TbZ3HzvFEto
(Counter-Strike 1.6 and Condition Zero (except Deleted Scenes) has "assertion" errors when Half-Life is updated. For 1,6 you can select the beta version which still works.)
Update: Also forgot to mention that Half-Life: Source is unlisted, and to get it you need to buy Valve Complete Pack. At least Valve knew it sucked lol.
(11/30/2023) Update: Counter Strike 1.6, Condition Zero (and Deleted Scenes) got fixed by Valve to support the 25th Anniversary Edition update.
TuffNuts
If you haven't played Half-Life yet, I implore you to play it. The game is not only a landmark for videogames in terms of technology and stroytelling, but it's also really fun. It's full of cool little details and it's such a blast to play. No time like the present to experience a milestone in gaming history.
lalakikenji
I already played Half-Life, at least a fraction of it. I had the 1998 version and Source in my laptop before I got the Steam version.