The unknown picture environment

The old picture environment, introduced by Leslie Lamport into the LATEX kernel almost 20 years ago, appears to be neglected in favor of more modern and powerful LATEX packages that eliminate all drawbacks of the original environment. Nevertheless it is still being used behind the scenes by a number of other packages. Lamport announced an extension in 1994 that should have removed all the limitations of the original environment; in 2003 the first version of this extension appeared; in 2004 the first stable version was released; in 2009 it was actually expanded with new functionality. Nowadays the picture environment can perform like most simple drawing programs, but it has special features that make it invaluable.