Libsamplerate
Libsamplerate - also known as Secret Rabbit Code (SRC) - is Erik de Castro Lopo's excellent audio resampling library. It was optionally available in Audacity up to and including version 2.1.0 if you compiled Audacity from source code.
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Audacity 2.1.0 and earlier contained code to use Libsamplerate for resampling, but it was not enabled by default due to licensing issues. Instead, Audacity first used a library called libresample based on Julius Orion Smith's Resample algorithm, then from 2.0.3 to 2.1.0 offered Libsoxr as default resampling library, with both libsamplerate and libresample being compile-time options.
From Audacity 2.1.1 onwards, only libsoxr is offered.
Until 2.1.1, libsamplerate could be used if you compiled Audacity from source code at the command-line, and configured --with-libsampletate. On Windows building with Visual Studio, you could #define USE_LIBSAMPLERATE in win\configwin.h.
If you use libsamplerate, you should not install extra plug-ins in Audacity unless they are licensed under the GPL or some license compatible with it.