I loved Scheme in the programming languages concepts class I took several years ago. Ever since reading what Paul Graham has to say about Lisp, I've been intending to go back and pick Scheme up again and see if it'll improve my programming in general.
Are there any well-known works of software written in Scheme? Open source packages? Websites?
It is (was?) used as a scripting language in some open-source packages software, such as The GIMP. There are also some GNU projects that use Guile (a Scheme implementation) as their scripting language.
I'm pretty sure ITA Software uses Scheme for most of their algorithms.
Some programs are not directly written in scheme but provide scripting capability through a scheme interpreter. Some examples are the gimp and the window manager sawfish.
See this SO question 'are-there-people-using-scheme-out-there' for some answers.
The Jak & Daxter series of games were written in a Scheme-like custom language called GOAL. A lot of Schemey features like garbage collection and dynamic list allocation had to go out the window to make it run in constant memory and time, but it's still recognizably LISPish. The GOAL compiler was written in Commmon Lisp.
Two that I am aware of personally: Whilst working at a telco, we installed a middleware product, basically a message router, that was written is Scheme, and used "schemelets" to identify and translate messages. That was my first exposure to it.
The second was a certain suite of compilers and IDEs (I can't mention the company or product for various reasons but they came from before the age of visual tools :-).
Also, emacs makes heavy use of its own dialect of lisp called Emacs Lisp. That is pretty related to Scheme, since Scheme itself is a dialect of Lisp.