Ned Wright’s Cosmology Tutorial and FAQ pages are a great source of knowledge
The Astronomical Society of the Pacific’s The Universe in the Classroom has a huge archive on all sorts of astronomical topics, often with classroom activities to go along with the information. Perfect for the budding astronomer.
NASA’s Imagine the Universe Ask an Astrophysicist site with a huge FAQ list.
Material for the NSW HSC Astrophysics topics
COSMOS an Encyclopedia of Astronomy by Swinburne Astronomy Online
Solar activities using real data to explore the Sun with the SOHO observatory. And these from NOVA Labs or STEREO. Then there is a whole week of activities to spend on the Sun during Solar Week.
Calculator for scale distances of the Solar System
Hawai’i Space Grant Consortium’s Exploring Planets in the Classroom with lesson plans and student worksheets
NASA’s Night Sky Network activities
A range of Astronomy activities from the Melbourne Planetarium