Maemo/Meego. I know there is Sailfish still around, but things would had been very different today if Nokia had put all its weight on it back then.
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Hit ctrl-f and typed Meego as soon as I saw this thread, hoping I'd be the first. Alas.
The N9 was literally a vision from an alternate timeline where a mobile platform from a major manufacturer was somehow hackable, polished, and secure. Favorite phone I've ever owned and I used it until it started to malfunction.
Had a Jolla for a bit, too. It was nice to see them try to keep the basic ideas going but unfortunately it was a pain in the ass to use thanks to their decision to go with a radio that only supported GSM/EDGE in the US. Had to carry around a MiFi just to give it acceptable data service.
I think the idea with Jolla is that if Nokia ever did an about-face, they were ready to be reabsorbed and get things back on the right track. Unfortunately, though we do once again have a "Nokia", it's just another Android white label with no interest in maintaining its own leading-edge smartphone platform.