Are there any sites like CodeAcademy for other fields, i.e. who specialize in math, languages, finance, etc. (no “generalists” like Khan)?Khan Academy is the most famous—it has over 3,000 videos covering topics from maths and science to economics and history. It’s aimed at kids but I know of plenty of adults who learn from there as well. It’s also a non-profit, and all of the content is completely free.
For language learning my wife has been having a really good time with Duolingo—she uses the iPhone app but they have a web version as well. It’s also free—I think their business model revolves around getting students to crowdsource translations as part of their learning exercises.