A frequently overlooked (also by me) Scheme implementation is the CHICKEN Scheme Compiler. What's special about this implementation: in addition to the interactive interpreter, there is a compiler that produces portable C and compiles it into loadable modules using a C compiler. This makes this compiler particularly good for integrating C libraries. In principle, this is still quite similar to Gambit-C, another Scheme implementation that uses C as an intermediate language.
But Chicken goes beyond Gambit-C in terms of generated C code - the system is explicitly designed to be mixed with C, while Gambit-C simply uses C as a portable assembler. In Chicken, the FFIs (Foreign Function Interfaces) are much simpler. This is evident in finished interfaces to various databases such as metakit (used in the Python Desktop Server), PostgreSQL, and sqlite.
In addition, Chicken has gained a nice infrastructure of network-installable extensions with the Eggs - with web server, database, and many other delicacies. This of course helps immensely in programming - I have come to love such an infrastructure of ready-made code with MZScheme, Python, and Perl.
Chicken also compiles under Mac OS X. At the moment, the compiler is running in the background for me.