Interior storm windows are a cheap means of improving the energy performance of windows. This is a design I came up with a number of years ago, which can be made for around $1 per square foot, and save that amount in fuel costs within somewhere between 4 months and 3 years (depending on existing windows, and local climate). In addition, they improve comfort levels, air leakage, and outside noise levels. If you can do only one thing, this could well be your best bet.
Blowing cellulose insulation into wall cavities is often cited as a job for professionals only, however I feel it is well within reach of competent do-it-yourselfers. This is a description of the job that I did (with help) to fill some holes in my insulation coverage. Blowing cellulose insulation into an attic is even easier (but make sure you seal all the air leaks first).
My kitchen range has a large hood with a fan that does not keep air or heat in very well. This is a description of how I insulated the duct above the hood, to try to improve performance, comfort, and reduce condensation.
The basement bulkhead (aka Bilco) door is one of the biggest energy holes in a house. In my experience, they all leak heat, and air and most leak water as well. Here is how I seal mine up for the winter.
A natural approach to keeping bees, this hive design can be made very cheaply from scrap wood. My total money outlay for this project to date is, $43.
Bees are currently under a lot of stress, particularly in the US, and where they are heavily commercially exploited, or insecticide use is prevalent. Individuals making up a hive or two, can, if done well, help the bees by giving them a place which is not driven by the desire to extract as much money from them as possible. A more natural approach reduces disruption of the bees, and encourages people to be aware of the effect they are having on bees.
This is a simple design for a sturdy wooden box. As a replacement for plastic tubs or many cardboard boxes, it is a neat project. One can be made from 12' of 1"x12" lumber, plus some dowels and rope.