Hold down the home and lock button at the same time.
If (for whatever reason) the power button isn't responsive, you can let the phone battery deplete completely. When your iPhone eventually turns off, charge it back up. It will turn on, while plugged in, in about four minutes.
Pro-tip: This is actually good practice for best iPhone battery performance anyway!
Be.ware. Did you know that hitting the volume buttons may turn the iPhone alarm off, instead of snoozing it? A lot of users experienced this issue after upgrading to iOS 8. You may need to go to Settings > General > Reset > Reset All Settings to fix it.
Found a lone iPhone? Press and hold the home button to activate Siri. Ask, "Whose phone is this?" and contact information will come up, as long as the owner has set their contact info. Now you can return the device to its rightful owner.
GOOD JOB, YOU ARE SUCH A GOOD PERSON.
Are you a light sleeper? Meet your salvation. In Settings > Do Not Disturb, you can set a scheduled time to silence all notifications, including calls and texts.
Take a close look at Allow Calls From, which will allow incoming calls from only "Favorites" or a contact group. You can also enable repeated calls within three minutes to go through.
The typing experience *sucks* on every single TV streaming device. If you're spending the night in and searching for something to watch, download these free mobile apps. They allow you to type quickly with the keyboard on your phone, instead of clicking the remote a million times: Amazon Fire TV, Remote by Apple, Roku, and Chromecast.
It's called AirDrop, and when it works, it's golden. You can only use it to send media (photos, links, and contacts) to other iPhones. The best part about AirDrop is that it sends photos really quickly. It's faster than text or email.
To use it, turn on both Wi-Fi and Bluetooth. Have your friend do the same, then ask him or her to open Control Center (swipe up from the bottom) and tap AirDrop to enable wireless sharing. In Camera Roll, select all the photos you want to share. Tap the share icon, and your friend's iPhone should appear at the top. Tap to share with AirDrop and the photos should appear in your friend's Camera Roll immediately.
You could also use it to troll unsuspecting strangers with photos of space sloths.
Open the chat, tap Details in the upper right, select Leave this conversation.
So that when you're driving and using Siri hands-free, she'll call the right person, instead of your ex or something.
Open Siri by long pressing the home button > say "Call Nicole Nguyen" (or whoever), then when Siri responds with the incorrect pronunciation, reply with, "That's not how you say that." She'll then prompt you to say the name correctly.
Disclosure: An iPhone 6 review unit was loaned to me by Apple for the purposes of this post.