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