How to run iPhone emulator WITHOUT starting Xcode?

Assuming you have Xcode installed in /Applications, then you can do this from the command line to start the iPhone Simulator:

$ open /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/Applications/iPhone\ Simulator.app

(Xcode 6+):

$ open /Applications/Xcode.app/Contents/Developer/Applications/iOS Simulator.app

You could create a symbolic-link from your Desktop to make this easier:

$ ln -s /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/Applications/iPhone\ Simulator.app ~/Desktop

(Xcode 6+):

$ ln -s /Applications/Xcode.app/Contents/Developer/Applications/iOS Simulator.app ~/Desktop

As pointed out by @JackHahoney, you could also add an alias to your ~/.bash_profile:

$ alias simulator='open /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/De‌​veloper/Applications/iPhone\ Simulator.app'

(Xcode 6+):

$ alias simulator='open /Applications/Xcode.app/Contents/Developer/Applications/iOS\ Simulator.app'

(Xcode 7+):

$ alias simulator='open /Applications/Xcode.app/Contents/Developer/Applications/Simulator.app'

Which would mean you could start the iPhone Simulator from the command line with one easy-to-remember word:

$ simulator

The easiest way without fiddling with command line:

1. launch Xcode once.
2. run ios simulator
3. drag the ios simulator icon to dock it.

Next time you want to use it, just click on the ios simulator icon in the dock.

In XCode 7+ the location is now

/Applications/Xcode.app/Contents/Developer/Applications/Simulator.app

Run it from the command line

$ open -a Simulator

Hope that helps somebody