Let's start with the most common Hard Reset methods and go from there:
Note: the different types of methods are numbered, method 1, method 2, method 3, etc.
1. When the phone is powered off, press and hold the Volume Up and the Volume Down keys both at the same time, then press and hold the Power key until a test screen that shows some available options appears, usually takes about 15-20 seconds. When that screen pops up you can let go of the keys.
Press the Volume Down key to navigate down through the options until it highlights FACTORY RESET then press the Power key to select it.
2. Power the phone off all the way, press and hold the Volume Down key and press than release the Power key, still holding the volume down key for about 10-15 seconds. When you see some additional options pop up on the screen you can let go of the keys.
Press the Volume Down key to navigate down through the options until it highlights the reset option; it usually says FACTORY RESET, then press the Power key to make the selection.
3. While powered off, press and hold the Home key, while continuing to hold the home key power the device on by pressing the Power key.
Once the Android Recovery screen appears release the Home key, then while on the Android Recovery screen, press the Volume Up and Volume Down keys both at the same time.
If/when you're on the Android System Recovery screen release all the keys, and then use the Volume down key to navigate down to the "wipe date/factory reset" option. Once it's highlighted press the Power key to select it.
Once you see the "Confirm wipe of all user data?" Press the Volume down key to highlight "Yes-delete all user data", then press the Power key to select it.
4. Power the device off, then press and hold the Home key and press and release the Power key (while still holding the Home key).
From the Android Recovery screen, press the Search key.
Tap to select wipe data/factory reset then select Ok (located in the lower left of the screen). Note: if screen is not responsive use the Volume keys to scroll and the Camera key to select.
Select Yes - delete all user data, and then select OK (again lower left).
Select "reboot system now" and press OK.
5. This is becoming a common hard reset method and can be used to factory reset Samsung's more recent Galaxy smartphones such as the Samsung Galaxy S III, the Samsung Galaxy S4, the S5, the Samsung Galaxy Note the Note II and Note 3, etc.
With the device powered off press and hold the Volume Up key the Home key and the Power key all at the same time.
When the phone vibrates (or you see the GALAXY Logo) release the Power key but continue to hold the Volume Up and Home key.
Continue holding the Volume up and Home key until the "Android system recovery" screen appears.
Use the Volume buttons to scroll to and highlight the "wipe data/factory reset" option, and then use the Power key to make the selection.
Once the factory reset is finished you may use the Volume and Power keys to highlight and select the "Reboot system now" option and the hard reset will be complete and your cell phone will be nice and squeaky clean.
6. Here is an alternate reset for a Motorola Atrix still running on the 2.2 Android OS...
While the phones power is off, press and hold the Volume down button and press and release the power button (note: if it doesn't work after all the steps listed try holding down the power button along with the volume down button in this first step).
A "Fastboot" screen should pop up, press the Volume down key to cycle through the choices until you get to "Android Recovery", then press the Volume up key.
Wait for the "triangle!/Android". Once it pops up you'll need to tap on the bottom right corner of the screen (note: it may take a few attempts). A menu should pop up.
Tap on "wipe data/factory reset", and then tap OK. Another confirmation screen will appear. Tap Yes and then OK.
After the data is cleared the "reboot system now" option should appear and is highlighted by Default. Tap OK.
Note: this type of reset is common on the 2.2 (Froyo) before the tech geeks wised up and removed the need to use the touch screen.
7. This reset is for the same phone, the Motorola Atrix, but one that's running on the 2.3 OS.
Power off the phone, press and hold the Volume down button and Power button simultaneously while powering on the unit.
The "Fastboot" screen should pop up. When it does press the Volume down key to cycle through the choices until you get to "Android Recovery", and then press the Volume up key to boot in this mode.
Wait for the "triangle!/Android" screen to pop up. Once it does press both the Volume up and Volume down keys simultaneously. A Recovery Menu should pop up.
Tap on the Volume down key until "wipe data/factory reset" is highlighted, and then tap the Power button. Another Confirmation screen will come up. Tap on the Volume down key until it highlights "Yes" and then tap the Power button to make the selection.
After it's done make sure "reboot system now" is highlighted and then tap the Power button.
In this method you don't have to use the touch screen any more. So let's say your touch screen isn't responding therefore you can't navigate the phone or go into your settings and do a regular factory data reset. Well now on the 2.3 there is hope. Just power off the phone and perform these steps and the reset should go through.
8. Here is a hard reset for a Sharp Android Smartphone.
While the device is off, hold the Sym button (it's located on the keyboard) and the Power button simultaneously.
Once the Android boot screen appears highlight the Wipe data/factory reset option by using the volume buttons. Then use the Return key (on the keyboard) to actually make the selection.
Scroll down to Yes - delete all user data and then press the Power key to confirm.
Once it is done select reboot system now and press the Power key to confirm and reboot your handset.