Cross platform, Free Software, almost all features you can think of, graphical and in the shell: Learn once, use for everything.
» Get Emacs «
Emacs is a self-documenting, extensible editor, a development environment and a platform for lisp-programs - for example programs to make programming easier, but also for todo-lists on steroids, reading email, posting to identi.ca, and a host of other stuff (learn lisp).
It is one of the origins of GNU and free software (Emacs History).
In Markdown-mode it looks like this:
More on emacs on my german Emacs page.