In previous iterations of Apple's OS X, the option key gained its notoriety as a modifier for simple keyboard commands such as volume adjustment. In Mountain Lion, the Option key gets a whole new set of responsibilities and when used in conjunction with toolbar commands, can provide some very useful information ranging from aiding troubleshooting to outright productivity boosts. Let's look at what the Option key does with some popular toolbar icons (items added by the use of the Option key are enhanced in red outlining).
On its own, the volume icon is pretty straightforward: you can increase or decrease the system volume. When you hold the Option key down while clicking on this icon, you'll get a lot of the same input and output selections available in the System Preferences pane. Choose from multiple inputs, outputs and if you have a 13" MacBook Pro like I do, you can choose the primary function of the single audio port on the left side of the unibody.
Upon initial inspection, the Airport (Wi-Fi) menu will show you a list of open and closed broadcast access points using the 802.11 protocol and will allow you the ability to turn on and off the power to the wireless chipset. When used in conjunction with the Option key, the menu displays technical information about the current SSID you're connected to, including information like the transmission channel, wireless band and type of security the access point utilizes. There's also a handy link to the Wi-Fi diagnostic application that is normally buried in the
/System/Preferences/Core Services directory - an app which is very handy for troubleshooting common issues with wireless connections.
Speaking of wireless connections, Bluetooth is a technology used to connect an Apple Wireless Keyboard and Magic Mouse (and even your BlackBerry). You can get basic Bluetooth connection information in the toolbar menu by holding down the Option key. The version of Bluetooth firmware, Bluetooth chipset name (usually your computer name) and MAC address will all be outlined.
Those of you with a portable Macs know that your productivity lives and dies by the health of your battery. In addition to showing you basic information such as the amount of power left in your battery, use of the Option key here will offer another cool extra: the ability to see the overall health of your battery. This is handy if you think your laptop just isn't holding the same charge as it once did. A good reference for additional battery health info is published by Apple on their website.
Those of you running in an enterprise environment may have something enabled by your network administrator called Home Sync. This is used if you're a mobile network user and either switch between multiple Macs in your workplace or can be used as an alternative to a full-blown Time Machine backup, syncing user preferences and documents - essentially anything in the ~/yourusername directory - between a Mac server's Open Directory and a client computer. By using the Option key here, you gain the added functionality of resetting the sync counter, which isn't very handy under most circumstances. I could only see it being used if the last sync day/time information isn't displaying correctly.
Mountain Lion's newest feature - the Notification Bar - can be quite handy for getting alerts from your Calendar, Mail, Messages, Twitter, Facebook… the list goes on and on. But what's a user to do if you want to temporarily mute the notifications? Sure you can swipe/scroll up on your mouse and click the off option for alerts and banners, but that only serves to permanently disable that feature.
You guessed it: press the Option key and clicking the Notification Bar icon enables the hidden option allowing you to resume alert and banner notifications tomorrow. Having that instruction enabled when you turn it off is handy for temporarily disabling these sometimes-productivity killers that ruin your focus when you're in the zone. A handy way to tell if this is enabled is that you'll notice the Notification Bar icon turn from black to gray when you have the Option key held down and click.