Valve
The Source Engine was developed in the years following Valve's initial licensing of the Quake engine for Half-Life in the mid-1990s.
The strengths of the Source engine reflect the focus the Seattle developer has placed on its own games: high-end graphical effects, facial and character animation, the integration of in-game physics, effective networking code and support for a wide range of PC hardware.
Unlike other middleware companies, Valve seems to have taken a measured approach since it started licensing the engine in 2004, working with a small group of developers rather than the large publishers.
This may be because of Source's PC-centric nature. Wii and PlayStation 3 are supported but licensees are warned such ports will require additional custom work.
Yet, for PC and Xbox 360 development, Source offers a fully featured solution. The renderer supports dynamic shadows and HDR, the latter even on DirectX 9-class hardware, while indirect radiosity is supported using a distributed solver and then baked into textures.
Other engine components include a unified material system covering textures, physical behavior and fracture properties; network-enabled physics; and a squad-based AI system. Development tools include the Hammer level editor, the Faceposer character animator and an audio suite.
Source
Features: DirectX 10 HDR renderer with advanced shaders; material systems; particle engine; networked physics engine; pathfinding and navigation system; Hammer level editor; facial and character animation tool; Faceposer
Platforms: PC, Xbox 360 (PlayStation 3 and Wii available but not officially supported)
Integration with Other Technologies: plug-ins for 3ds Max, Blender, Cinema 4D, FragMOTION, LightWave 3D, Maya, Milkshape 3D and Softimage XSI
Cost: Available on request
Released Games Include: Counter-Strike: Source (Valve), Dark Messiah of Might and Magic (Arkane), Half-Life 2 (Valve), Portal (Valve), Team Fortress 2 (Valve)
Games in Development Include: Left 4 Dead (Turtle Rock Studios), Mabinogi Heroes (Nexon), Postal III (Akella), Salvation (Black Wing), The Crossing (Arkane)
source.valvesoftware.com

Valve's Half-Life 2
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On a related note, I'm pleased to find that there's more support for Linux than I previously thought. I'll have to look at a couple of them closer.
> It's safe to say Epic's Unreal Engine 3 is the current,
> de facto industry standard middleware
It's safe to say that it's currently the most popular 3rd party engine for AAA current-gen titles. That's not really the same thing as being a de facto industry standard. You wouldn't say the Ford Focus is the de facto standard for UK cars, just because it has the largest market share. By definition a de facto standard has to be so completely ubiquitous that anything other than it seems odd - that's not Unreal 3's status.
Like: Unity, Unigine, StemCell, NeoAxis, Quest3D.
And much more, there are Game Engines very good and the prices is more low.
Shouldn't this article be called "List of expensive commercial engines" ?
Why not the Nebula engine?
Or ... well.... the list is long: http://en.wikipedia.org/wiki/List_of_game_engines
Game engines have come a long way in the last few years and the commercial engines that had the market to themselves need to realise that they face competition and need to restructure thier licensing. the engine with the best tools and licensing will make a lot of money.