If you acquire a used phone or want to wipe a device before selling it, you'll need to do a hard reset. HardReset.info is a simple site with one purpose: Give you detailed instructions on how to do a hard reset on that phone, whatever model it may be.
It's not difficult to find out how to hard reset a phone -- the information is usually a Google search away -- but HardReset keeps it all in one place, and includes screenshots to show you where the buttons are, what screens you should see, and what the process looks like. The site itself is simple enough: Just type in the name of your phone in the search bar, or click "Device" in the upper right corner to browse by manufacturer or mobile carrier -- you'll find even obscure devices are in the database.
Hit the link below and bookmark it, it might come in handy the next time you need to reset your device, and as always -- make sure you back up your phone's data before doing a hard reset.