License: Lesser GNU Public License

As LGPL was designed for libraries (such as SDL) that the creator wanted to make available to non-GPL applications (rather than changing the app to a GPL compatible license), it is unusual for a standalone game to use it.

While the main purpose was to include other non-proprietary licenses (like BSD) there's nothing barring the use of LGPL code with completely proprietary apps (which is very common with proprietary games). It is important to note that LGPL code must be kept as a separate entity from the apps they are used with (kept as .dll or .so files for example). LGPL code cannot be reused or compiled into a non-LGPL or non-GPL app. However, LGPL code can be changed to GPL code in part or whole.