Cube (Portal 2) - Valve Developer Community

Valve Corporation, Half-Life Wiki

Source - Valve Developer Community

Portal 2 - Valve Developer Community

Source - Valve Developer Community

Steam's top-rated game sees free prequel delayed as Valve reviews it

Valve is Seemingly Working on an Unannounced Game with “Never

Very new to Hammer, started making a non-eucledian map! : r/Portal

Portal 2 - Valve Developer Community

Portal 2: Community Edition - Valve Developer Community

Valve Hammer Editor - Valve Developer Community

Portal 2: Community Edition - Valve Developer Community