Other
The Mathematical Elements of Euclid
An extensive discussion of the Elements, which contains all the geometrical knowledge available in the Hellenistic period.
University of St. Andrews (UK)
University of St. Andrews: Non Euclidean Geometry
The University of St. Andrews in Scotland provides an excellent site explaining Non-Euclidean geometry.
Thomas Jefferson National Accelerator Facility
Jefferson Lab: It's Elemental Periodic Table of Elements
This page displays the Periodic Table of Elements and allows you to click each individual element for a page that displays in-depth information about that particular element. There are also links to many different games and learning...
Clark University
Clark University: Euclid's Elements
A review of the 13 books of Euclid's "Elements" text can be found here. Each of the chapters are broken down into definitions, propositions, and common notations that are used throughout this overview.
Other
Oak Ridge National Labs: Erbium
A summary of the abundances of the isotopes of erbium. A great resource for chemistry students!