The History of Geometry (Wikipedia.org) - has an interesting survey of the development of geometry throughout Ancient, Greek, Medieval, and early Modern eras. There is especially good information Hindu, Chinese, and Islamic geometry during the Middle Ages. Also, there is an associated Wiki page with a good Timeline of Geometry.
Euclid’s Elements (David Joyce, Clark Univ) - a well-organized online version of the complete Elements of Euclid, Books I-XIII. All of the figures are illustrated using the Geometry Applet, so that all figures can dynamically change if Java is enabled.
The Origins of Greek Mathematics (Don Allen, Texas A&M Univ) - a summary of the historical evidence about the origin of geometry, with overviews of the main schools of Greek mathematics.
Ancient Greek Mathematics (Don Allen, Texas A&M Univ) - an interesting collection of essays about Greek geometry from Thales to Pappus.
Archimedes (Chris Rorres, Drexel Univ) - a comprehensive site about the greatest of the Greek geometers.
Mathematicians of the 17th and 18th Centuries (David Wilkins, Trinity College, Dublin) - biographies of the major mathematicians (Descartes, Fermat, Pascal, Euler, Monge, etc) and many of their less well-known contemporaries.
Online Euler Archive (MAA.org) - This is the largest online collection of Euler's papers and books in the world. Euler wrote more than 800 papers in his life, and many are associated with geometry. Most of the documents are photocopies of the original publications. Euler (1707-1783) is considered to be the greatest modern mathematician. Nevertheless, 300 years after he was born, most of his clearly written work can be read by undergraduate math majors, and be a fertile source of ideas for them.
The Historical Development of Algebraic Geometry (PDF) - by Jean Dieudonne (MAA.org) - an amazing online document that starts with the Greeks, and meticuously traces the path leading to modern 20th century Algebraic Geometry (up to 1970). This is both a great history of modern geometry, as well as an invaluable guide to this modern field of mathematics. But, it is not written for the lay person, and is meant for professional mathematicians. The author was one of the top researchers in the field.