Mathematics

• Kevin Cowtan's Book of Fourier (What's the Fourier transform of a duck?)
• Mathematical constants
• Prime numbers
• Knots and links
• Knot Atlas
• Encyclopedia of integer sequences
• Crystallographic topology
• Sprott's Fractal Gallery
• Jim Hoffman's geometry pages
• Gallery of Computation
• Math with Legos
• Acme Klein bottles

Dinosaurs!

• University of California Museum of Paleontology
• National Museum of Natural History (Smithsonian) Paleobiology Department
• Dino Dictionary
• Simulated sounds of Parasaurolophus

Other Extinct Critters

• Orders of trilobites
• Conodonts
• Mazon Creek Fossils (including Tullimonstrum)
• Oceans of Kansas Paleontology
• Palaeontologica Electronica

Biology Stuff

• Pfiesteria piscicida (the "cell from hell")
• Lemurs!
• Rotifers
• The Tree of Life

Earth-bashing Asteroids

• Risks of asteroid and comet impacts
• Chesapeake Bay impact
• Earth impact database
• Solar system impacts calculator
• Near-Earth asteroids
• NEAR encounter with Asteroid Eros
• Hayabusa encounter with Asteroid Itokawa

The Solar System

• The Nine 8 Planets
• NASA Human Spaceflight
• Planetary Photojournal
• JPL planetary exploration missions
• Messenger to Mercury
• New Horizons to Pluto (and 2014 MU₆₉)
• Cycloical features on Europa
• New moons of Jupiter (and other outer planets)

The Universe

• Searching for extrasolar planets
• Extrasolar planets encyclopedia
• NASA PlanetQuest website
• NASA Exoplanet archive
• Kepler spacecraft
• Hubble Space Telescope
• Chandra X-Ray Observatory
• Ned Wright's cosmology tutorial
• Cosmic Background Explorer (COBE)
• Wilkinson Microwave Anisotropy Probe (WMAP)
• Planck Spacecraft (European mission to study the cosmic microwave background)

If You Really Have Nothing Better To Do

• You could check out the Death Clock.
• If your anticipated lifespan extends past 2036, here are some things to worry about.
• To get really depressed, read John Baez's page about the end of the universe.