Ask HN: Great Codebases You've Worked With?

It's so hard to tell the difference framework hype, truly promising ideas, and actual good architectures. I'm looking for tales of setups that _really worked_, whether it's a massive formally-verified distributed system, or a backend completely implement in the DB layer, or some bizarre excel-like-live-updates-but-for-streaming-data framework invented inside a massive fintech co (that one's based on a vague memory of an HN thread I saw earlier this year).

Anyway, we could all learn from testimonies of truly great software, where "great" should be taken to mean "effective", not necessarily "elegant".