NASA
Nasa: The Geiger Counter
Site from NASA explaining the construction and use of a Geiger counter. The site has links to many related topics, such as plasma, ions, and electrons.
Other
Geiger Counter: Radiation Monitoring
This article, by a engineer and physicist, features graphs made with a Geiger counter in order to measure the Radon levels in his house.
BBC
Bbc: Gcse Bitesize: Radioactive Emissions
This lesson focuses on radioactive emissions including radioactive decay, half-life, using half-life, how to calculate net decline, a table showing how the count rate of an isotope might change over time, and a link to a test.