The Interactive NGC Catalog Online This is the interactive NGC (and IC, and Messier) catalog atSEDS, based on the famous NGC 2000.0 by R.W. Sinnott of Sky Publishing Corp., who also created the electronic version used by this online service (thanks toSky Publishing for permitting us this kind of use). An excellent reference.
You can participate in a crowd-sourced search for exoplanets (planets orbiting stars other than our Sun) by looking at data from NASA's Kepler satellite. Wicked cool and participatory science.
Notes from an astronomy course at the University of Tennessee, Knoxville (USA).
"In this semester of Astronomy we shall concern ourselves primarily with the Solar System. As an introduction to that, we shall consider the historical development of our modern picture of the Solar System."
This animated GIF shows images from the Hubble telescope and is useful for demonstrating the vast size of the universe, the number of stars it contains, and the possible number of planets out there.
The Exoplanet Data Explorer is an interactive table and plotter for exploring and displaying data from the Exoplanet Orbit Database. The Exoplanet Orbit Database is a carefully constructed compilation of quality, spectroscopic orbital parameters of exoplanets orbiting normal stars from the peer-reviewed literature, and updates the Catalog of nearby exoplanets.
A complete history of the Universe, from the Big Bang up to now. Too much to list here, really, but it includes loads of resources and lessons that can be used to teach astronomy, earth science, and life science, with links to agriculture and modern society. Pretty darn cool overall.