Can anyone help me? I'm trying to get an arduino-powered locker door to unlock and open the door for me... A knock sensor maybe? What should I use?
Any help would be appreciated! Thanks...
Use a numeric keypad. Once the right code is entered a solenoid pops the lock and a nice motor or pneumatic cylinder swings the door open. Or better yet spring load the door. Then it pops open as soon as the lock is released.
I've seen recently a secret knock lock, where you had to knock a certain rhythm to unlock it.
That would be a cool thing to have in a locker.
You see a kid standing in front of a locker, then--- bum bada bum bum...BUM BUM*-- it opens on its own!
*(shave and a haircut)
In school, it wouldn't be long before someone figured out your Secret knock.
How about RFID? or a magnetic sensor?
No matter how you implement it, make sure you can get into the locker when the batteries run dry 
Unless of course you have mains power in your locker.
YIF,
The Instructables web site has recently featured two different Arduino based door locks:
Secret Knock Detecting Door Lock
Arduino RFID Door Lock
Perhaps these will guide you in the right direction.
What should I use?
Common sense.
If you are in "grade school" (ie, high school or lower); quit putting expensive junk in your locker (leave it at home), carry the rest around in your backpack, and keep an eye on your pack.
If you are at a University or College, I am sure someone somewhere sells insurance for dorm rooms and contents, similar to renter's insurance (and if not, that may be a interesting and fruitful niche product you might want to get into if you can find the capital to start it up!).
If you live in an apartment, get renter's insurance. If you own a home, make sure your homeowner's policy is adequate to cover the contents in case of loss. Invest in a home security system. Get to know your neighbors - maybe even become "gasp!" friends with them! Have a trusted friend watch your house, water your plants, and feed your pets.
Above all, document everything you consider "valuable". Keep your receipts (I have boxes of receipts dating back over 10 years). For your really valuable stuff (cash, jewelry, guns, etc) - consider a personal safe, or a safety deposit box (a small size safety deposit box runs about $100.00 a year depending on the bank - it is worth every penny, depending on what you have to put in it).
Yes, this is boring; no, it isn't "sexy", and it doesn't involve using an Arduino anywhere. It is, though, what is called "common sense" and "being an adult" (17 year olds and under need not apply).
One other final bit of advice: keep a low profile and look "poor"; don't go all flashy on things, shop a lot at goodwill, purchase as many items used as possible (especially cars). Bank your savings. You'll end up with a good nest egg, and people won't ask to borrow money off of you. People will avoid hitting the beater you drive because it looks like you don't have insurance (the bigger and uglier the beater, the better - this doesn't mean it has to run poorly, though - learn to fix and tune your own car).
On the other hand, if this is just some fancy "do it to do it" project, then - carry on...
/yeah, I know - I'm such a downer...
After you read cr0sh's post and realize that you still want to toss $50-80 of electronics into your locker...
What about a bluetooth module. Pair it with your phone, that way anytime you're within range your locker opens up. When you walk off, it locks.
Yeah, but what if he walks down the hallway and someone is standing right in front of his locker?
What if he knew the secret?