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.
I loved my N900, and my N800 before that, and I would have loved to have seen successors. Ultimately, I ended up switching to Android because I was tired of things only available as apps. Since then, web technologies have gotten better, and it's become much more feasible to use almost exclusively websites.