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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.
Also, to add to this: if you haven't played Portal and Portal 2, then you need to do that now. Like possibly even before HL
Was thinking of adding those but wanted to keep it HL specific. But yes the Portals are must plays. Pretty much perfect games. I remember watching the ending of 2 over and over because it was so dope.