Projectile is a project interaction library for Emacs. Its goal is to provide a nice set of features operating on a project level without introducing external dependencies. For instance - finding project files is done in pure elisp without the use of GNU find.

This library provides easy project management and navigation. The concept of a project is pretty basic - just a folder containing special file. Currently git, mercurial and bazaar repos are considered projects by default. If you want to mark a folder manually as a project just create an empty .projectile file in it. Some of projectile's features:

• jump to a file in project
• jump to a project buffer
• multi-occur in project buffers
• grep in project
• regenerate project etags

Just drop projectile.el somewhere in your load-path. I favour the folder ~/.emacs.d/vendor:

(add-to-list 'load-path "~/emacs.d/vendor")

You can enable projectile globally like this:

(require 'projectile)
(projectile-global-mode) ;; to enable in all buffers

To enable projectile only in select modes:

(add-hook 'ruby-mode-hook #'(lambda () (projectile-mode)))

If you're an Emacs 24 user or you have a recent version of package.el you can install projectile from the Marmalade repository.

Projectile is naturally part of the Emacs Dev Kit. If you're an EDK user - projectile is already properly configured and ready for action.

Here's a list of the interactive Emacs Lisp functions, provided by projectile:

• projectile-jump-to-project-file (C-c p j)
• projectile-grep-in-project (C-c p f)
• projectile-replace-in-project (C-c p r)
• projectile-switch-to-buffer (C-c p b)
• projectile-multi-occur (C-c p o)
• projectile-regenerate-tags (C-c p t)
• projectile-invalidate-project-cache (C-c p i)

• Traversing the project directory programmatically (instead of using something like GNU find) is not very fast. On the other hand - it's portable. Unlike find-file-in-project, projectile's jump-to-file will work on any OS.
• To compensate for the lack of speed - a cache is created when a project is traversed. That cache is not automatically updated (presently) so you might want to invalidate it manually from time to time (or disable it completely for small projects).
• Some operations like find/replace depend (presently) on external utilities such as find and perl.

None at the moment. You could be the first!