Constructing the Universe
This is the best source of sacred geometry information that I know of. Michael Schneider's book, A Beginner's Guide to Constructing the Universe, and his workbooks are fabulous primers for teaching yourself about sacred geometry.

Bruce Rawles' Sacred Geometry Site
This was the first site I found on sacred geometry several years ago, and it still remains one of the best. Bruce Rawles, it's architect, was definitely ahead of his time. As you explore the site, you'll find that he not only explains the concepts behind Sacred Geometry, but also provides you with exercises to make your own. If you are looking for an easy to follow primer on Sacred Geometry, I'd suggest starting here.

Geometry in Art & Architecture
The links below offer an excellent course in the history of Sacred Geometry in art and architecture.
Unit 1: Introduction
Unit 2: The Golden Ratio and Squaring the Circle in the Great Pyramid
Unit 3: Pythagoras and Music of the Spheres
Unit 4: Number Symbolism
Unit 5: Polygons, Tilings, and Sacred Geometry
Unit 6: The Platonic Solids

The Quintessential Guide to Sacred Geometry
Sacred geometry can be found everywhere in the world around us. Since the time of the Ancient Egyptian Pyramids, men have been creating architecture based in forms found in sacred geometry. There are patterns in nature as well. The world and the universe around us are filled with sacred geometry. From seashells to the human body, from the cosmos to the atom, all forms are permeated with the shapes found in sacred geometry.

Pedagoguery Software
Poly is a shareware program for exploring and constructing polyhedra. With Poly, you can manipulate polyhedral solids on the computer in a variety of ways. Flattened versions (nets) of polyhedra may be printed and then cut out, folded, and taped, to produce three-dimensional models. There's nothing better to help you visualize the platonic solids, and to open your eyes to the magic of geometry.

Natural Geometry and Holy Nouns
One of my favorite sites for looking at the rotations of the platonic solids. If you click on any of the two dimensional objects shown, a three dimensional version will appear that you can then turn around with your mouse. This gives you a very clear sense of the geometrical form. The site also links the platonic solids to the Kabalah and the Tree of Life.

LandryArt.com
Ken Landry is an incredible polyhedra maker. Using balsa and paper as his medium, he's created some stunning models, often incorporating the mind bending geometries of M. C. Esher. Two of his largest polyhedra are included in the "Escher Extrasensory Museum show that is currently traveling throughout Japan.

Science in Ancient Artwork
A site with an incredible number of teachings about mathematics and sacred geometry, and their connections to ancient art...a veritable lifetime of learning to be had here.

Flower of Life Research
Drunvalo Melchizedek, Teacher and Founder of the Flower of Life Workshops, has developed a powerful meditational practice, called Merkaba, that goes hand in hand with a deep understanding of Sacred Geometry. This web site offers some wonderful material on Sacred Geometry, and the Merkaba Meditation.

Friar Magnus Wenninger
Fr. Magnus Wenninger, OSB is a mathematician, polyhedra builder and member of Saint John's Abbey in Collegeville, MN. On this web site, you'll find images of some of his remarkable polyhedra constructions... very beautiful, as well as astonishing accomplishments.

Harmony and Proportion
A site that will teach you much about Harmony, Proportion, and the relationship between music, space, and mathematics. An excellent primer.

Sacred Geometry
Some further discussion on Sacred Geometry, linking it to crop circle designs and to the Kabalah.

San Graal School of Sacred Geometry
An introduction to a world of information about the Universe and its relationship to Ancient Teachings. Also see more San Graal links here.

Kheper
This is a website containing over a thousand pages, dedicated to evolution and transformation, whether individual, planetary or cosmic. This is s large project that is still growing, and will probably continue to do so over time. It has some very useful information to anyone interested in consciousness, evolution and transformation.

Metaforms
A catalog of sacred geometric three dimensional wire forms for sale--very beautifully made.

The Spirit of Maat
Aa monthly on line magazine editted by Drunvalo Melchizedek, spiritual teacher and author of the Flower of Life books.

Surfing the Acpocalypse
A Sacred Geometry page on a much larger site that includes many excellent links to other Sacred Geometry sites.

The Interactive Qabala
For those interested in the Qabala (also spelled Kabbalah), this Interactive Qabala project takes advantage of hypertext to make immediate the connections between many disciplines including Jungian metapsychology, Yoga philosophy, astrology, Tarot, and Qabala. The Tree of Life is a map; with HTML, it becomes clickable - the correspondences can be shown or hidden instantly, in vibrant, full color. A truly exceptional site.

Mid-Atlantic Geomancy
Another excellent introduction to and overview of Sacred Geometry and its ties to sacred sites around the world. Put together by Mid-Atlantic Geomancy, this site is accessible to a large audience. It is better organized than many of the other sites listed here, and so is recommended for beginners, though it also contains information that will be of great interest to those already familiar with Sacred Geometry

The Meru Project
Based on 30 years of research by Stan Tenen into the origin and nature of the Hebrew alphabet, and the mathematical structure underlying the sequence of letters of the Hebrew text of Genesis. A wonderful and powerful site, that helps one grasp the depths to which geometry informs and generates our universe.