Recs.
Updated
Vim is a powerful, portable, keyboard based text editor. Being text-based, vim is lightning fast, with an incredible set of features developed over its multi decade existence.
SpecsUpdate
Pros
Pro Extensible
Lots of plugins, many of which are beneficial to writers, especially with reference to markdown. There are plugins for file browsing, automatic folding, outlining, distraction-free and focus modes, live markdown previews, notes, bookmarks, wikis, todo lists, and much more.
Pro Lightweight and fast
When compared to modern graphical editors like Atom and Brackets (which have underlying HTML5 engines, browsers, Node, etc.), Vim uses a sliver of the system's memory and it loads instantly, all the while delivering the same features. Vim is also faster than Emacs.
Pro Keyboard-based, mouse-free interface, and trackpad support
There's no need to reach for the mouse or the Ctrl/Alt buttons again. Everything is a mere key press or two away with almost 200 functions specifically for text editing. Vim does support the mouse, but it's designed so you don't have to use it for greater efficiency.
Versions of Vim, like gVim or MacVim, still allow you to use the mouse and familiar platform shortcuts. That can help ease the learning curve and you'll probably find you won't want to (or need to) use the mouse after a while.
Pro Has multiple distinct editing modes
Interaction with Vim is centered around several "modes", where purpose and keybindings differ in each.
Insert mode is for entering text. This mode most resembles traditional text entry in most editors.
Normal mode (the default) is entered by hitting ESC and converts all keybindings to center around movement within the file, search, pane selection, etc.
Command mode is entered by hitting ":" in Normal mode and allows you to execute Vim commands and scripts similar in fashion to a shell.
Visual mode is for selecting lines, blocks, and characters of code.
Those are the major modes, and several more exist depending on what one defines as a "mode" in Vim.
Pro Extremely portable
Vi/vim exists on almost all Unix-like platforms. It's the de-facto Unix editor and is easily installed on Windows. All you need to make it work is a text-based connection, so it works well for remote machines with slow connections, or when you're too lazy to set up a VNC/Remote Desktop connection.
Cons
Con Difficult learning curve
You'll spend a lot of time learning all the commands and modes supported in Vim. You'll then spend more time tuning settings to your needs. Although once it's tuned to your needs, you can take your .vimrc
to any machine you need and have the same experience across all your computers.