Computer password recovery is a subject of very large scope. Here we will break it down into various areas and help direct you to the Wiki resource that best fits your specific need.
Windows Operating System: User Password
Hiren's Boot CD
You can boot from the Hiren's Boot CD Restored Edition and use it to reset the password for your Microsoft Windows installation. Versions of Windows supported depends on what release of Hiren's that you are using.
Microsoft Boot CD
Windows 7 password recovery can be accomplished with a Windows boot CD or boot media.
1. Boot off the Windows disk and select the “Repair your computer” option from the lower left-hand corner.
2. Follow through until you get to the option to open the Command Prompt - then choose Command Prompt.
3. Type the following commands:
move c:\windows\system32\sethc.exe c:\windows\system32\sethc.bak copy c:\windows\system32\cmd.exe c:\windows\system32\sethc.exe
4. reboot
5. Once you get to the login screen, hit the Shift key 5 times, and you’ll see an administrator mode command prompt.
6. Again you have to type in a command at the command prompt:
net user nicolep password
Change "nicolep" to your user name and "password" to your choice of passwords
After you reboot you will be able to login again. Although not necessary, you should boot from the Windows boot disk again, and follow steps 1, 2 and then reverse step 3 by typing:
copy c:\windows\system32\sethc.bak c:\windows\system32\sethc.exe
This restores disability mode in the event that you use the disability feature. It is not a necessary part of Windows.
Offline Windows Password editor
Offline NT Password and Registry is a free utility that you can place on a boot CD and use to recover your password.
This software runs in a command-shell (DOS) environment.
Android Device: Google Password
See Change the Google Account Password on an Android Device
Linux Operating System
Resources here include Emergency Boot Recover Root Password or Fix Mounting Issue.