Other
Triumf: Environment, Health & Safety
An extensive index containing links to many sites describing radiation measuring instruments. A comprehensive website on the topic.
Georgia State University
Georgia State University: Hyper Physics: Geiger Counter
Contains some pictures and a brief description of how a Geiger Counter functions.
Other
Rathaus: Measuring Radiation
Guides the reader through the origins of some of the units of radiactivity measurement, including the curie, the rad, and the becquerel.