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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.
Maybe it's because I played them too late, but while I mostly had a blast playing HL2, the first one never clicked for me.
I know, it's been very influential and new when it released, but it was still quite straight a FPS game. Whereas HL2 is like a crazy theme park of different ambiences and mechanics.