• Easily run multiple instances of NetLogo in OS X
• Double click nlogo files to open them in a new instance of NetLogo, so that they don't close the model you're working on!
• Drag and drop one or more nlogo files onto NetLogo Launcher to open them all at once

Go to System Preferences and select Security & Privacy. Under Allow apps downloaded from: is set to Anywhere.

This is the directory that contains NetLogo 5.1.0.app, NetLogo 3D 5.1.0.app, and so forth (the version numbers may be different). NetLogo Launcher should work with most NetLogo versions as long as the app name looks like NetLogo #.#.#.app.

In finder, go to your NetLogo installation directory. Drag NetLogo Launcher.app to the dock. Now you can click on the NetLogo Launcher icon to open a new instance of NetLogo, or drag and drop nlogo files onto it to open them in new instaces of NetLogo.

This makes it so that when you double click on an nlogo file, it won't replace the model you have open!

Go to any nlogo file. Ctrl-click (or right click) on the file and select Get Info. Under Open With, go to Other.... Find NetLogo Launcher.app, select it, check Always Open With, and hit Add. Finally, back in Get Info, click Change All....

After that, double clicking on any nlogo file should open it in a new instance of NetLogo!

If you want to build NetLogo Launcher yourself, you need the command line version of Platypus. I recommend getting this from Homebrew. With Homebrew, just run

brew install platypus

Once Platypus is installed, just run make in the NetLogo Launcher directory to build a new app file. make debug will create an app that leaves its output open. make clean will delete the generated app file.