Google Sketchup is a Free, easy to use, and intuitive 3D modeling tool.
Because it is free, I've found Google Sketchup to be a great tool to use in the classroom when teaching my high-school classes about engineering drawing. Some of my students can pick up google sketchup right away, while others have to go through a few of the online tutorials before they start to get the hang of it. At any rate, at the end of the day it's a valuable tool for them and I usually require they use it in one of their reports or presentations. If you'd like Sketchup pro for your K-12 school, Google already has educational licenses for most states, as well as case studies and examples of using Sketchup in the classroom.
I don't just teach it, I use it too! I've used google sketchup at home when I'm planning out a new woodworking project to help me conceptualize how various pieces will go together and look before I begin with the sweat and blood of actually building things. It's actually really fun to be able to stretch or shrink parts as you change your conception of what you'd like to build. Google Sketchup, along with the 3D wear-house of pre-drawn models, is a great tool if you're trying to remodel or rearrange your home. You can drag pieces of furniture around and knock out walls, all from the comfort of your laptop!
For research, I've used Google Sketchup to design new experimental apparatuses. Once they're drawn, it's easy to throw a picture from sketchup into publications as experimental setup figures. If you look at my Experiments in Fluids paper, you'll notice that a couple of my figures were done in Google Sketchup! With a copy of Sketch-up Pro, these sketches can be exported to a number of formats, including autocad and .eps files, which is awesome if you'd like to import them into another program. I should note that a student licence of Sketchup is only $49/yr, although I am usually able to get by only using the demo version of pro.
Give the free version of sketchup a try, I hope you have fun playing around with it!