Mountain Lion interferes with MAMP localhost on port 80
After upgrading to Mountain Lion OSX on my Mac, I found that MAMP wouldn't start Apache if you were on port 80. Changing it to 8888 or another port would allow it to start.
In previous versions of OSX, web-sharing allowed you to see your localhost from other computers on the same network running it's own version of Apache, disabling this would prevent it's own version of Apache from starting up and occupying port 80 automatically. Apparently in Mountain Lion, Apple decided to remove the option of web-sharing from the System Preferences, instead opting to automatically start Apache on port 80. As a result, when MAMP attempts to start it's version of Apache, it gets denied.
How do you fix this so MAMP is running it's on Apache version on port 80? Simply from terminal, you'll need to stop the first instance of Apache with the command line:
$ sudo apachectl stop
Once the first instance of Apache has stopped, port 80 frees up and now all you need to do is restart MAMP Apache.
That's it. Hopefully future versions of MAMP will automatically detect a conflict and opt to stop the preloaded version of Apache before starting it's own.