For vim there is a filesystem explorer called NERDtree: https://github.com/scrooloose/nerdtree
Surely an alternative or superior package exists for Emacs?
Personally, I prefer the ECB's tree mode. http://ecb.sourceforge.net/
You should also check out the CEDET stuff. http://cedet.sourceforge.net/ They contain a lot of other useful stuff that you can use for similar development needs.
Try out lusty-explorer. It's a well done plugin. Get it here
The Sunrise Commander seems to be a well written package, and it has a tree extension. For a screenshot of the tree extension in action, follow this link.
I prefer dirtree (screenshot below) by Ye Wenbin. It feels pretty close to NERDTree / the TextMate drawer.

It took me a few minutes of trial and error to figure out the dependencies, so take a look at the install instructions (and a few minor edits to the source) I've got in this repo.
You can try NeoTree .
https://github.com/jaypei/emacs-neotree
Screenshot:

(Shameless reanswer from A good project tree browser for Emacs? - Stack Overflow)
I just now did a word search for "explore" in package-list-packages, and discovered project-explorer. Seems to fit exactly what I want today (I don't code hardly, but getting a grip on the structure of my Jekyll site).
Keys include TAB for folding and unfolding directories. Open files with RET or f, but with prefix, it will prompt nicely for which window, and even from there allow you to decide to use window or open up a new one to any side (I didn't find the prompt string in the package code, so it seems to leverage built in Emacs functionality nicely; indeed it looks like dired even).
It's available on Melpa and Marmalade, and packaged description points to sabof_project-explorer · GitHub.
I include the site's image for convenience:

I don't use projectile or helm, but it has some integration.