Scheme is used mainly for teaching purposes. That's one reason there hasn't been a standard module system until the (controversial) RSR6 one: when writing small programs for homework it's unusual to need lots of big libraries.
Scheme started out as an experiment to add Smalltalk-like OOP to Lisp. It eventually changed the macro system, added call/cc
(and dynamic-wind
, dynamic-unwind
), and grew quite a bit.
Aside from the GNU projects using Guile as a scripting language, I don't know of any production systems using Scheme. I do know of a handful of production systems using Lisp, though (Orbitz has Lisp code in its reservation system).