Geochronology

Berkeley Geochronology Center - Non-profit scientific research institution dedicated to establishing the history of the Earth, its various inhabitants, and its interactions with the rest of our Solar System, throughout the 4.6 billion years of our Planet's existence.
http://www.bgc.org/

Web Time Machine - Geologic time scale with links for each time interval.
http://www.ucmp.berkeley.edu/help/timeform.html

Geological Time - Lecture notes on different methods of measuring geologic time.
http://rainbow.ldeo.columbia.edu/courses/v1001/geotime2.html

Geochronology Working Group at Vienna University - Uses radiometric dating and classical geochronological methods to constrain the timing of geological processes, primarily in the Eastern Alps, the Carpathian - Pannonian region and adjoining area, and the Bohemian Massif. Includes descriptions of several isotopic techniques.
http://www.univie.ac.at/geochron/

Geologic Time - USGS on-line publication about the relative and radiometric geologic time scales, age of the earth, index fossils, and related topics.
http://pubs.usgs.gov/gip/geotime/

Strontium Isotope Stratigraphy - Describes this method, used primarily to date marine sediments, and provides access to the global seawater strontium curve and a user-friendly table for converting 87Sr/86Sr values to numerical ages.
http://www.ucl.ac.uk/geolsci/research/geochemistry/sis/