Text based data-interchange format, for use in games and other applications where performance and size is important. Project started in 2009 as a replacement for XML, you should be using JSON at this point.
SimpleD was originally written in VB.Net, then ported to C#, PHP and Java.
While developing I wrote a custom tester that tested syntax of the implementation, and performance as I optimized.
C#'s single-file can easily be dropped in a project and is Unity3D compatible, but does not contain serialization witch most Unity Dev's are use to.
The PHP port was used in a few websites, and has not been thoroughly tested.
The Java port was never really used or tested, it was built as a programming exercise.