I love
The Encyclopedia of Earth, which has great scientific articles for all ages. It helped get me through my (homeschooled) G12 biology and geology, actually. It's also got videos now, which it didn't when I was really heavy into it.
The University of Adelaide has all of the Greek and Latin Classics, and a lot more besides, as free e-books.
For biology, and mycology in particular,
Dr. Fungus is fantastic, and has a lot of pretty disgusting pictures of mushrooms besides.
In Geology and Plate Tectonics and whatnot, a pair of excellent sites are the
IRIS World Seismic Monitor and the
Volcano World News. These are bookmarked on my computer as "What's Shaking?" and "What's Blowing?"
For Astronomy and other celestial things, there's nothing better than NASA. Of their many and varied sites, I am particularly fond of the
SOHO Solar Observatory and
The Hubble.
And for Botany, the
UBC Botanical Gardens Forums are an excellent resource, as is
Plants For A Future. And for bugs,
BugGuide!