Unity

On Windows at least Unity games have certain recognizable traits, these are the easiest to recognize.
* "Game Name.exe" file and "Game Name_data" folder in base folder.
* "Game Name_data" folder has "resources.assets" and "sharedassets0.assets" files which are very large.
* "Game Name_data/Resources/" has "unity_default_resources" and "unity_builtin_extra" files.
* "Game Name_data/Managed/" has "UnityEngine.dll" among others.

Should not be confused with Canonical's the Unity Desktop.

Platform support:
* Windows 2000 or newer (32-bit or 64-bit)
* Mac OS X (32-bit, 64-bit, or fat binary)
* Wii
* iOS
* Web browsers (via Unity web player plugin, with Unity 2.6)
* X360
* PS3
* Google Native Client (with Unity 3.5)
* Linux† (32-bit, 64-bit, or fat binary)
* Flash (discontinued in 5.0+)

Supports rendering through both Direct3D and OpenGL

Integrated middleware:
* PhysX
* Vorbis
* Theora

Currently a Unity 4 or 5 license costs $1,500 USD or $75 monthly for 12 months minimum (so $900 minimum). Upgrades from Unity 3 or 4 cost $750.


† Linux is officially supported with Unity 4. Unity 4.1 added a Linux deployment kit. The Linux deployment kit is for distros with cooperating developers. So far Valve has helped provide SteamOS deployment and Canonical provides for Ubuntu deployment. Facebook games tend to work to a playable degree with Linux, but Facebook has become involved in a a Facebook deployment kit to better ensure compatibility (Facebook servers run on Linux).

Unity's own website documents how 'official' Linux support can be added to projects by developers who have paid for a Unity 3 license. They can use the Demo version of Unity 4 then backport to full Unity 3 (involves a few mouse clicks, no additional expense). It is unknown if any major developers have taken advantage of this but is known that several developers have instead opted to upgrade to a full Unity 4.x license to provide Linux support. This is amazing considering the initial cost of this option was over $11,000 dollars (its only $750 these days). Presumably, the same technique can be used for other platforms. Though, as of yet, there is no official documentation on the Unity site so such backports can be presumed to be unsupported.

Mutiny is an open source replacement for Unity. It can be used by developers to make any game they would use Unity for, code can be transfered both ways between the two. However, there are some minor code readability and resource efficiency improvements in Mutiny that will not translate directly to Unity and must be implemented manually or using macros. End users can use Mutiny to run Unity games that have not been ported to their platform, provided that other libraries, middlewares and such that a game uses are also available on their platform.
Mutiny should not be confused with the replacement desktop for of Canonical's Unity that is also called Mutiny. Nope, not kidding.