This is not a tutorial — it is a guide on how to get these tools up and running as fast as possible. All of them have excellent documentation, so you really should be digging deeper to find out what else they can do for you.

1. Install Mac OS X's missing packages with Homebrew

Homebrew is a package manager for Mac OS X (kind of like yum or apt-get if you've ever been on a Linux machine). Install it along with 2 packages which will be needed for compiling Ruby versions:

ruby -e "$(curl -fsSL https://raw.github.com/Homebrew/homebrew/go/install)"
brew install openssl autoconf

2. Use a version manager for managing and switching between different Ruby versions

The world is split into fans of rbenv and RVM. They both do an excellent job of letting you install different Ruby versions simultaneously and switch between them on a per-project level.

As I prefer rbenv, I recommend you install it along with ruby-build (a plugin which lets you compile and install Ruby versions with one simple command):

brew update
brew install rbenv ruby-build

Now installing Ruby 2.1.0 is as simple as rbenv install 2.1.0. To make this version your default, use the command rbenv global 2.1.0.

Before prematurely starting to manage your gems with the rbenv-gemset-plugin, please read my article about managing multiple Rails versions.

3. Let pow serve your application

pow is a zero-configuration Rack server, which means that running your local Rails applications will be made ridiculously easy. Install it with curl get.pow.cx | sh and every time you create a new Rails application, simply symlink it to your .pow-directory:

cd ~/.pow
ln -s /path/to/myapp

This will make your application available at http://myapp.dev and http://www.myapp.dev — without firing up Webrick or messing around with ports. If you make any changes to your app which require a restart, simply touch /path/to/myapp/tmp/restart.txt and pow will restart this project's server before the next request.

4. Test your app from different devices with xip.io

xip.io runs a custom DNS server on the internet which provides wildcard DNS for any IP address. Sounds hacky but makes testing your app from different machines a breeze:

Assuming your local IP is 192.168.0.10, grab your iOS or Android device, make sure you're in your WLAN and access your application at http://192.168.0.10.xip.io.

This even works with subdomains (http://subdomain.192.168.0.10.xip.io) and natively joins forces with pow (see step 3), so you can prepend the URL with your application's name (http://subdomain.myapp.192.168.0.10.xip.io), which makes testing multiple apps simultaneously very easy.