Text Editors
Visual Studio Code
Available for Windows, Mac, Linux
Cost: Free
Style: GUI
Visual Studio Code is a source code editor developed by Microsoft that includes support for debugging, embedded Git control, syntax highlighting, intelligent code completion, snippets, and code refactoring.
Atom Editor
Available for Windows, Mac, Linux
Cost: Free
Style: GUI
Atom is a free and open-source text and source code editor for macOS, Linux, and Microsoft Windows with support for plug-ins written in Node.js, and embedded Git Control, developed by GitHub.
Both Atom and Visual Studio Code are hugely popular and the majority of web developers that you'll meet will probably be using one or the other. Both feature multiple cursors and they share many of the same keyboard shortcuts. Also much like Visual Studio Code, Atom has a rich ecosystem of packages that you can use to customize your editor to your heart's content.
Emacs
Available for Windows, Mac, Linux
Cost: Free
Style: Command-Line or GUI
Website or Aquamacs (for OS X)
Emacs is an open source text editor that's been around since the 1970s. Along with Vim, it's one of the most popular Linux text editors.
Emacs is often described as an operating system because even in a clean install there are several included applications that you wouldn't expect inside a text editor, like a news reader, several calculators, a number of games, file encryption/decryption, and a package manager for plugins written in Emacs Lisp.
All Emacs commands exist in the same namespace so it's not uncommon to differentiate commands by having chains of keystrokes like C-x C-f (Control - x, Control - f) to open a file. Because of the incredible customizability, it's among the editors with the steepest learning curves. You will most likely want to customize it: installing plugins, trying them, testing for conflicts, uninstalling the ones that have conflicts, and repeating.
Vi/Vim
Available for Windows, Mac, Linux
Cost: Free and open source
Style: Command Line or GUI
Vim, or Vi IMproved is the other text editor in the Unix Editor Wars.
Vim runs anywhere that standard C can run and is often in the base install for most Linux and non-Windows systems including Mac OS X. It also offers a fairly robust tutorial to learn how to use it. Learn it once and you can use it everywhere.
For experienced students (everyone else, it's ok if you don't understand this right now): Vim relies on modes, or scopes, when certain commands are applicable. In the command mode, the user can move around a file or execute commands. For instance, in insert mode, you can edit a file. While you are creating a HTML file (and are in HTML mode), you might be able to expand html:5 into the boilerplate for an empty HTML file.