Erlang due to its error handling philosophy and FFTW due to the following:
Question 4.1. How does FFTW work?
The innovation (if it can be so called) in FFTW consists in having a variety of composable solvers, representing different FFT algorithms and implementation strategies, whose combination into a particular plan for a given size can be determined at runtime according to the characteristics of your machine/compiler. This peculiar software architecture allows FFTW to adapt itself to almost any machine.
For more details (albeit somewhat outdated), see the paper "FFTW: An Adaptive Software Architecture for the FFT", by M. Frigo and S. G. Johnson, Proc. ICASSP 3, 1381 (1998), also available at the FFTW web page. [1]
[1] http://www.fftw.org/faq/section4.html#howworks
Edited for one more comment:
The last paragraph also reminds me of Alan Kay with its focus on the interaction between systems and not their specific logic.