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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.
They've admitted to cancelling ideas before, getting to various stages of production before going back to the drawing board, but always (and appear to still) insist that it is in development on some level. That's why Newell's responses to questions about hl3 are usually some form of "we have nothing new to share." Valve doesn't like sharing until they're in the final stages of development, and hl3 has never made it that far.