Farcaster

I think the quiet revolution in user experience, mainly through Account Abstraction, has been the most mind-blowing development. It doesn't grab headlines like a new token, but it's solving the single biggest barrier to mainstream adoption.
It's a game-changer because it finally makes Web3 wallets work like modern accounts. Things like social recovery (instead of losing everything with a lost seed phrase), paying for gas fees with any token, or even having apps sponsor transactions for you are becoming standard. It removes the scary, technical hurdles for regular people and makes using a dApp feel as simple as using any other app on your phone. Without this, Web3 would have always stayed a niche hobby.