Viewing Saved WiFi Passwords

Android

Open the settings app
Select "Network & Internet"
Select "Wi-Fi"
The Wi-Fi network you are connected to is shown at the top of the page. If this is the network you're looking for tap the settings icon.
Otherwise, scroll to the bottom of the page and selected "Saved networks"
Scroll through the list of saved networks until you find the one you're looking for and tap the padlock icon
Tap on the share icon
Enter your pin
Either scan the QR code or read the password from the caption at the bottom.

Old versions of Android may not have this feature.

Windows

Open an administrative command prompt

Run netsh wlan show profile to see a list of SSIDs

C:\WINDOWS\system32>netsh wlan show profile

Profiles on interface Wi-Fi:

Group policy profiles (read only)
---------------------------------
    <None>

User profiles
-------------
    All User Profile     : MWireless
    All User Profile     : Gbomb
    ...
    All User Profile     : SuperDuperSketchyCorp_388
    All User Profile     : MGuest


C:\WINDOWS\system32>

Run netsh wlan show profile <profile name here> key=clear and look at the Key Content field

C:\WINDOWS\system32>netsh wlan show profile Gbomb key=clear

Profile Gbomb on interface Wi-Fi:
=======================================================================
    ...
Security settings
-----------------
    Authentication         : WPA2-Personal
    Cipher                 : CCMP
    Authentication         : WPA2-Personal
    Cipher                 : GCMP
    Security key           : Present
    Key Content            : <password here>
    ...

C:\WINDOWS\system32>

This does not work for all types of networks (for example WPA2-Enterprise)

C:\WINDOWS\system32>netsh wlan show profile MWireless key="clear"
    ...
Security settings
-----------------
    Authentication         : WPA2-Enterprise
    Cipher                 : CCMP
    Authentication         : WPA2-Enterprise
    Cipher                 : GCMP
    Security key           : Absent
    802.1X                 : Enabled
    EAP type               : Microsoft: Protected EAP (PEAP)
    802.1X auth credential : User credential
    Credentials configured : No
    Cache user information : Yes
    ...

Encrypted profile information is stored at

C:\ProgramData\Microsoft\Wlansvc\Profiles\Interfaces\

Linux

List the saved connection profiles: ls /etc/NetworkManager/system-connections/
View saved password: cat /etc/NetworkManager/system-connections/<SSID>.nmconnection
To view all the saved passwords at once: sudo grep "psk=" /etc/NetworkManager/system-connections/*