markdowntutorial - Useful to get up and running with markdown. I liked the bite sized lessons and the browser based inetractive lessons. Multiple languages are also supported. however, it is hard to navigate to specific parts of longer lessons. It also did not cover how to format codeblocks .
Regexone - A great Site for learning regular explessions. I loved the bite sized lessons and the interactive exercises which can be completed from the browser itself. It also has a helpful table of contents which you can use to navigate toa particular lesson. Can be done in 30 min. Useful as a reference as well.