Secret Rabbit Code (aka libsamplerate) is a samplerate converter for audio. One example of where such a thing would be useful is converting audio from the CD sample rate of 44.1kHz to the 48kHz sample rate used by DAT players.
SRC is capable of arbitrary and time varying conversions, from downsampling by a factor of 256 to upsampling by the same factor. Arbitrary in this case means that the ratio of input and output sample rates can be an irrational number. The conversion ratio can also vary with time for speeding up and slowing down effects.