Crookes radiometer (~$8):
Mesmerizing, and the science is just incomplete enough to keep everyone guessing how it works. Turns out it won't work in a complete vacuum, nor at normal air pressure.
I use it to show the transformation of light energy into kinetic energy, cool enough, but for the true wackadoodles in your classroom, challenge them to figure out how to make it spin "backwards."
I'm so in love with my radiometers I keep one at home, too.
Class set of magnifying glasses (~$20):
Pictures in textbooks pale compared to a dead bug found on the windowsill, which pales next to a live slug under a magnifying glass!
Magnifying glasses are ridiculously easy to use, and double the complexity of the visual world as soon as a child starts using one.
Large empty pretzel jars ($5 but you get to eat the pretzels):
I collect these for class--they make great terrariums, as well as cheap bookshelves (just lay something flat and sturdy on a couple of pairs). They can also serve as giant stocking blocks to demonstrate endergonic reactions (energy in, more elaborate order) and exergonic reactions (knock them all down with just a little activation energy).
I also use them to carry stuff in from the outside--snakes, pill bugs, elodea, slugs, centipedes, whatever.
Write a letter to your school board, your state Secretary of Education, and to the Emperor himself, Arne Duncan, asking them why your child can recite biochemical cycles yet has no idea what a wheat berry looks like, or why plants need water, or what causes the seasons to happen.
Demand that local, state, and federal school officials take the same standardized tests required of your children, and ask that their scores be posted publicly.
True science education takes time to observe, time to reflect, time to get things wrong before putting the pieces back together in a way that makes sense of the natural world.
To see a world in a grain of sand,
And a heaven in a wild flower,
Hold infinity in the palm of your hand,
And eternity in an hour.
Your child spends a lot of time in my classroom.
Help me make it worth her time.
Here in New Jersey, we have an end of course biology exam, that may or may not count. Thankfully my kids did reasonably well last year--though I expect, always expect, them to do better.