redsea is an RDS decoder for use with RTL-SDR tools. It can even decode RDS-TMC. Written in C++14, using the liquid-dsp library. The name is sort of a pun.
deinvert decodes the voice inversion scrambling used as a sort of privacy filter in some radios. Written in C++11.
bpm is a command-line tool to measure beats per minute just by hitting enter in the terminal. Written in Perl 5.
Okay, it's not all about signals:
kalevala-grep Match all input text lines conforming to the Kalevala trochaic tetrameter. Written in Perl 5.
markovtext Generate random chains (lines of text) based on sample text data given as input. Written in Perl 5.