Steam

Steam is a digital distribution, digital rights management, communications and multiplayer platform developed by Valve.

=Issues=

Cannot Find Steam (or similar)
Solution

 Exit Steam by clicking on 'Steam' -> 'Exit' Re-open Steam, on Windows 7 right click 'Run as Administrator' 

Error adding friend. Please try again.
Solution

 Block all communications with the player you are trying to add Unblock the player Add them as a friend again 