Editors and IDEs

For coding in Ruby you can use the default editor of your operating system. By the way, to be more effective in coding, it is worth to choose a source code editor with basic Ruby support (e.g. syntax-highlighting, file browsing) or an integrated development environment with advanced features (e.g. code completion, refactoring, testing support).
Here is a list of popular tools used by Rubyists:

• Linux and cross-platform tools:
  • Aptana Studio
  • Emacs with Ruby mode and Rsense
  • Geany
  • gedit
  • Vim with vim-ruby plugin and Rsense
  • RubyMine
  • SciTe
  • NetBeans
  • Sublime Text
  • Atom
• On Windows:
  • Notepad++
  • E-TextEditor
  • Ruby In Steel
  • Atom
• On Mac OS X:
  • TextMate
  • TextWrangler
  • Dash (documentation browser)
  • Atom