The first Half-Life title to be released since 2007. The last official game from Valve was Half-Life 2: Episode Two (2007).
A small porcelain figure of Scout from Team Fortress 2 can be seen sitting in front of one of the computers in Russel's Lab. In fact, the player can get an achievement by placing the figure on Russell's holographic map when he asks for an item to be placed. The achievement is called "Little Slugger".
The opening balcony scene has a radio you can interact with. When turning the knob on a certain channel, you can hear the same distorted trumpet music used in Half-Life 2, when seeing The G-Man briefly appear on an unplugged television.
A possible start to the jokes that Valve can't "Count to three." Other notable games that could have contributed are Half-Life 2 Episode 1 & 2, The Lab, Aperture Hand Lab (the previous 2 being VR demos) and Aperture Desk Job. There's even a fan music video called "Count to Three" (they even got Ellen McClain, the original GLaDOS, to be GLaDOS in this) on YouTube.