Initially released on Steam in 2006, the paid download lets you bring models from various games into an open sandbox and play with them accordingly. Since then, it's brought in over $3 million and given creator Newman financial stability. nope
"This game requires that you own a Source engine game such as Counter-Strike: Source, Half-Life 2, or Day of Defeat: Source, cautions the program's official description. "Click here for a full list of games that meet this requirement. Please note that Portal: First Slice and the bundled Half-Life 2: Deathmatch do not fulfill this requirement."
The half-price sale runs until September 21. Thanks to JediDwag for the tip.