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/Developer/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