I just received a catalog from one of my favorite companies - Montessori Services.
It helps me plan our course of study for the year. When I see something and think, "Oh, that's cool!" I buy it or look for something similar at Rainbow Resource.
Andrew built this bird feeder from a kit we purchased from them last year.
We've also purchased a prism, a jumbo rain guage and thermometer, a garden spider web frame, a child sized hammer, and books about weather, worms and light.