From the third best Scheme in the world - Gambit Scheme System - there is a new beta for version 4.0. Particularly interesting about Gambit-C is - besides the high performance of the code - the truly brilliant threading implementation. Normal Scheme continuations are used and then a dispatcher is placed on top. As a result, threads under Gambit-C hardly need any memory (on a 2G machine you can easily run over a million threads) and resources (and yes, the switching with a million threads is also quite decent). As a result, Gambit-C is the absolute number 1 for massive multithreading on single-processor systems - and web servers love many threads.
And before anyone asks: the best Scheme is of course PLT Scheme (Dr. Scheme) and the second best is Chicken Scheme - because Chicken Scheme still has the most impressive library of included code after PLT Scheme. Gambit-C could learn a lot here and include more libraries, because libraries are what really make the language usable. At the moment, it is still quite bleak for Gambit-C.
By the way, the license issue is finally resolved: you can now choose between LGPL and Apache license for Gambit-C, which should really make all license discussions obsolete.