education
The BGS has an interactive map of the Geology of the UK (Flash / Shockwave required). Outcrop distribution for different geological periods can be switched on or off and mousing over the information button for a period brings up a timescale for that period. Very useful for teaching stratigraphy. [Hat-tip: Highly Allocthonous Blog]
An introduction to metamorphic rocks from Prof. Stephen A. Nelson at Tulane University.
A brilliant strategy game based on finding oil using money left by a deceased Aunt. Uses a Java applet and comes linked with pages to do some reading before you start. From the American Geological Institute.
Some excellent Java applets that let you locate an earthquake using seismograph readings. The site may take some time to load but it is worth it.