Steam
Steam is the program used by VALVe Software in order to deliver game content quickly, securely, and always up to date.
One of the main reasons Steam was implemented was to prevent people from obtaining a cracked serial number and playing online for free, as this reduces profit for the software company.
Steam's necessarity is often disputed. Many see it as too slow, resource consuming, and unnecessary. Others see it as fast, always up to date, and extremely convenient.
Users that would like to get Steam may download it free of charge at http://www.steampowered.com , or for a direct link: http://steampowered.com/download/SteamInstall.exe