Emacs extensionsEdit

Show line numbers on the left-hand side of the buffer

• linum.el: http://stud4.tuwien.ac.at/~e0225855/linum/linum.html

Highlight whitespace errors on the fly

• whitespace.el: http://www.emacswiki.org/emacs/WhiteSpace

TextMate-style "snippets

• YASnippet: http://code.google.com/p/yasnippet/

Color themes

• http://www.cs.cmu.edu/~maverick/GNUEmacsColorThemeTest/

Indexing

• Not an Emacs extension as such, but used by Emacs to index symbols in source trees: Exuberant ctags: http://ctags.sourceforge.net/

You would create an Emacs-compatible tags index, tags, with: cd project_dir && /usr/local/bin/ctags -e -R

Rails

• Rinari

General enhancements

• ido.el, or IDO "interactively do" mode: http://www.cua.dk/ido.html – provides enhanced versions of C-x C-f (find-file) and C-x b (switch-to-buffer) commands
• ECB, or Emacs Code Browser (speedbar replacement): http://ecb.sourceforge.net/
  • CEDET, or Collection of Emacs Development Environment Tools, a dependency of ECB: http://cedet.sourceforge.net/