Forth is an elegantly simple and powerful programming language that has found its strongest niche in embedded real-time control systems. To give you a brief idea of its popularity, here is NASA's list of Space-Related Applications of Forth. Also, Open Firmware, which ships as OpenBoot (tm) on on over one million Sun Microsystems SparcStations(tm) and SPARCServers(tm) since 1989 and on Apple Computer's newest line of PCI bus-based Power Macintosh(tm) desktop systems.
I was lucky enough to get involved with the language when I was hired by Forth, Inc. in 1980 as a technical writer. While there, I wrote the book Starting Forth, which became a classic, and surprisingly thrust me into the position of being a Forth "expert." Amazingly it remains popular to this day, although it is once again out of print. (Except, apparently, in Russia, from whence I occasionally receive a small royalty check.)
I was especially lucky that Forth's inventor, Chuck Moore, worked at Forth, Inc at the same time. It didn't take me too long to realize that the best Forth programmers thought about problems differently -- and used a different software development process -- in more elegant ways than most other programmers at that time. It seemed their way of thinking was closer to the problem being solved. So I wrote a second book, Thinking Forth, to capture the Forth methodology. Although obsolete in some ways, it stands up pretty well today given that it is going on 30 years old. The book is based on interviews with many Forth programmers including Chuck, and research especially in the seminal work by David Parnas on encapsulation and data hiding (approaches which informed the movement toward object-oriented programming and are still being explained in new books).
It's fun to read favorable reviews by Adam Petersen and Ville Laurikari!