Euclid's Elements
http://aleph0.clarku.edu/~djoyce/java/elements/elements.html
This is a vast site that demonstrates theorems in Euclid's Elements by drawings and interactive Java Applets. The site has the complete text of this work in complete form. The creator of this page,David E. Joyce, in his introduction calls the Elements "one of the most influential works of science in the history of humankind." One enjoyable task might be to find Proposition 47 in Book I.
Math 131 Spring 1997
http://www.unc.edu/~rowlett/math131/index.html
Professor Russ Rowlett, who teaches at the University of North Carolina, has posted everything done in a math course on Euclidean and non-Euclidean Geometries with links to theorems and properties of Euclidean and Hyperbolic Geometries. One important thing here is how clearly the idea of ABSOLUTE GEOMETRY is presented. See the remarks on Propositions 1-28.
Susan Addington's Geometry Course
http://www.math.csusb.edu/courses/m129/euc_and_proofs.html
Valuable information here includes things about the Golden Ratio, the Euclidean concept of proof, and more on the Pythagorean theorem.
Byrne's Edition of Euclid
http://sunsite.ubc.ca/DigitalMathArchive/Euclid/byrne.html
Oliver Bryne, an English mathematician, uses color to illustrate propositions in the first six sections Euclid's Elements. His efforts to use color instead of words are fascinating. There is a link to Ralph Abraham's Visual Elements, which is an edition of the work with color pictures showing each proposition.