The Interactive Data Language from Research Systems Inc. A programming language
IDL is good for easily manipulating arrays of data and producing 2d and 3d plots and images. It is used by, amongst others, scientists working in space, atmospheric, astro, enviromental and medical disciplines.
It runs on Windows, Unix (incl Linux), Macs and VMS (although VMS support is on its way out and continued Mac support is currently uncertain). IDL code should be platform independent. The code looks a bit like FORTRAN and supports objects and the production of GUIs with Tk-like "widgets". Now provides a complete "Developer Enviroment" that looks like Windows regardless of your platform. This is great if you like Windows.
IDL is almost identical to PV-Wave. Both languages grew from the same source, but eventually split. IDL and PV-Wave are now developed separately. IDL is more widely used than PV-Wave. IDL is also very expensive.