Science

Using LCD and cellophane, you can inexpensively make your own 3D display. This is a great article going into the science behind stereoscopic viewing, as well as detailed instructions on how to create your own 3D display. other interesting projects (e.g. using 2 cell phone diplay images to create a 3D image) are also presented.

You don't actually see the cosmic rays, but this site shows you how to build a "Cloud Chamber" to view the cosmic ray's trail (which is still extremely cool). For more background information and pictures, take a look at this site.

Build an inexpensive seismometer using instructions and links from this site. The site mainly has descriptions of his projects without going into great detail regarding how to build a seismometer, however the links and the resources listed give you everything you need to build one yourself.

"It is simple, inexpensive, and fun to grow your own snow crystals, using little more than some dry ice, a plastic Coke bottle, and some styrofoam cups. This page describes how to set up this experiment, and what you can expect to see."
And other images

Here's a site built for students that shows you everything you need to know about building a model solar powered car. Besides instructions on how to build a model solar car, there are also instructions to build a mini wind tunnel.

Here's an article that details different setups for receiving weather satellite imagery to your computer. The first is an inexpensive, very basic setup, that produces some decent results. He progressively improves each system, until he gets some outstanding results from the final (and considerably more expensive) setup.

This site has very good instructions, along with detailed images showing how to build a high quality, fairly powerful, Dobsoninan telescope. For more information about John Dobson, you can read about him here

This isn't as much a project, as it is a pretty cool product. Take this Insta-Snow powder (it's actually a super absorbent polymer), add water, and a very realistic snow-like product is created. Now if someone took this powder, covered a hill in their backyard with it, and skiied or snowboarded down it, that would be a worthy GeekDIY project

The title may be a little misleading, it should say, "Find a Prime number with over 10 million decimal digits, and win 100K," that was a little too long though. The Electronic Frontier Foundation (EFF) is giving away $100K to the first person or group to discover a prime number with at least 10 million digits.
Now a lot of you might be wondering how you go about finding a 10 million digit prime number. One way is using GIMPS (The Great Internet Mersenne Prime Search). They offer a free client download that will search for primes using your machine. If your client finds the prime, current rules state that half will go to you and half to the GIMPS organization. BTW, the current largest prime record holder for a 7 million digit prime is held by a GIMP volunteer.

Using just a microwave and a bar of chocolate this article shows how you can measure the speed of light. She also covers the basic history behind measuring the speed of light, and some physics regarding light waves.