I could help with ideas on how to make a frequency meter !!
http://interface.khm.de/index.php/lab/experiments/arduino-frequency-counter-library/
https://www.pjrc.com/teensy/td_libs_FreqMeasure.html
http://www.avdweb.nl/arduino/hardware-interfacing/frequency-period-counter.html
Frequency could mean anything from detecting the colour of a sample of light to the coming and goings of the season. Just a rough idea of the range you are looking at would help.