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Half-Life 2 is currently 100% off for its 20th anniversary, plus a major update
(store.steampowered.com)
Bonus: it also seems that the episodes have been rolled into the base game. Full details of the anniversary update.
Sheeiit I absolutely loved the first one, even only having played it for the first time last year. I was blown away at how well it holds up. I do recommend trying it if you have even a morsel of interest. Just my personal 2 cents tho.