For most homeowners it’s their alarm or someone else in the household’s.
For renters, the alarm of someone in an adjoining unit
For farmers it might be a rooster, or someone else warming up the machinery
For suburbanites, the passing of the commuter train
For travelers, the wake-up call
For new parents, the baby’s hungry
For not-so-new parents, the children fighting
For military, reveille
For prisoners, watchtower sirens
For monks, the chimes
For me, it’s the furnace triggered by my new scheduling thermostat. Gotta love technology.
It’s always one of three things: my fiancee getting ready for work, my alarm clock, or (and my fury rises considering this) my IDIOT NEIGHBOR who always goes about finding his car in the parking lot every morning by hitting the panic button on his remote and following the sound of the alarm.
For me it’s needing to pee, backed up with an alarm clock. That or shame and regret. 
A 4-foot, 65 pound little boy diving on me. Anything less doesn’t work.
“Hello, front desk? Airman Doors, room 224 here. Right, about that wakeup call. I don’t suppose you have any children or midgets on staff by chance…” 
For some reason, I’ve never needed an alarm clock or anything to wake me up. I get up at 3:00 AM during the work week. My body just seems to know it is time to wake up.
The cat. Between 4 and 430 every morning.
Goats licking my bare feet.
It’s complicated.
The muzzle of a corgi in my palm.
Well, that or an angry Officer/Senior NCO, but they don’t count because they could wake the dead when they’re pissed off.
I have an elaborate system involving an alarm clock, a cell phone alarm, and snooze buttons, but usually it’s my bladder or my psycho kitty.
Terror.
Okay, either terror or one of four alarm clocks on a bad day. On a good day, terror just before any of the alarm clocks go off.
On a very bad day, all four alarm clocks can be shut off and/or placed in other rooms without me actually waking up.
I have no idea. I just wake up. I usually beat my alarm by anywhere from 30-90 minutes.
If I’m sleeping down the bottom of the garden, the light.
If I’m sleeping in my bedroom - the sheer blind panic that I have to go to work, which puts my body into a state of pumping adrenaline. Not pleasant.
Normally the sounds of my mom running around the house to get ready for work wakes me up. My bedroom door is right dab smack in the middle of the hallway connecting her bedroom door to the bathroom door. After anyone walks down that hallway enough times in a huge bustled panic, it wakes me up.
Other than those mornings when Mom needs up at 2:30 AM for work, mostly it’s my own motor that gets me going. When I was working, I got used to waking up around 6 each morning, so now I am preprogrammed to do so. Even when I do have the alarm set, I’m already awake long before it goes off.
I generally wake up about 1-2 minutes before my alarm goes off, just by reviewing what time I need to wake up before I go to sleep. I still set the alarm though, because it’s that ONE time I don’t wake up on my own that I’ll actually need to be somewhere!
If I’m camping at a festival and I want to get up post-midnight for the bonfire (but I need to nap to make it 'til dawn), I’ll drink three huge glasses of water before I hit the sack. The bladder alarm is the best there is, especially when snuggled in warm blankets with a lot of cold air between you and the bonfire! An alarm clock is too easy to turn off and go back to sleep.
The muzzle of a springer spaniel just barely tickling my nose as the wiggle of her back end is transmitted to the front.
7 Jack Russells bouncing on my bed.
My alarm. But it used to be that I just simply woke when I was ready. I’m hoping that soon I’ll have a job near enough to me that the drive can happen whenever I want, and I’ll go back to sleeping as long as I want.
Chihuahua standing on my head, usually.
The other day it was when my husband came in to see if I was alive. I’d been sleeping for 14 hours, and he thought I’d passed away.