Undoubtedly one of the most illusive and highly sought after games ever, Half Life 2: Episode 3, may have been in production since 2008 if concept art apparently leaked online is any indication.
The ValveTime.net Facebook page (the main site is currently unavailable) has posted what it reports are Episode 3 concept art. As you can see below the art work definitely does appear to have a Half Life theme running through it with Alyx, a Combine soldier and as well as other characters and environments.
You can also see more of the images here. Valve of course have never confirmed the existence of Episode 3 or Half Life 3.