It seems that Apple is taking a page from Google's book with their new "Hey, Siri" voice activation feature in iOS 8. This feature mimics the one found in Google Now, the voice-activated intelligent personal assistant available to just about all Android users. Apple's version allows you to bring up Siri with a simple voice command, but there's a small catch.
In order to enable this hands-free feature, head over to Settings -> General -> Siri and Allow "Hey Siri".
Your iOS device needs to be connected to a power source for the Voice Activation feature to work. If you're unplugged, nothing happens. This is a simple battery-saver move, but it only applies to older devices. If you're getting an iPhone 6 or 6 Plus, you will not have to be plugged in to use "Hey, Siri" mode.
UPDATE: Newer devices, like the iPhone 6s, 6s Plus, SE, and 9.7-inch iPad Pro, do not need to plug into a power outlet if they have iOS 9 installed.
Subsequently, with "Hey, Siri," Apple removed "Raise to Speak" as an option to start Siri, since it's basically the same thing, only now you have to actually talk to Siri like s/he's a real person. They probably should have left this feature in for older devices, since we have to be plugged in to use voice activation by itself.
Once you're connected to a power source, just say "Hey, Siri." I was able to bring up Siri pretty easily on the first try. I can see myself using it when I charge my phone in the car and need to get directions, send a text message, or make a phone call without having to take my eyes off the road.
Yay for safety.
To see what else you can do in Apple's latest mobile operating system, check out the Coolest Hidden Features of iOS 8, as well as the Best Hidden Safari Features, Top Hidden Features in Messages, and our other iOS 8 tips.