A home brew take in the Swamper project from Ham Radio for Arduino and Picaxw, using the Cypress library with updates from the book, but with a Nokia b&w display and a home brew PC board.
Arduino 2.4 GHz Spectrum Analyser
This article describes a simple spectrum analyser for the 2.4 to 2.5 GHz Wi-Fi band. It's based on the Cypress CYWM6935 Wireless Radio Module. Although the manufacturer doesn't recommend it for new commercial projects, it is still widely available from eBay suppliers and Farnell here in the UK.