Programming - C/C++
Professionally I'm a Delphi programmer, and have been since about 1995. For most of that time I've also used Delphi for personal projects, but along the way I've also dabbled in other languages, and the one I keep returning to is C++.
The main reason for this is that it is so widely used, and for years was virtually the de-facto standard program language. Its position has since been seriously challenged, but it is clearly going to be around for a long time, and it felt odd to be professional programmer with little C++ knowledge or experience.
Another reason for looking at C++ was my interest in 3D programming. Almost all of the significant 3D libraries and game engines use C or C++.
As a result, I've been playing with C++ on and off for years, and can usually put together a decent program, if only after a lot of head-scratching and reading of tutorials and reference books.