Materials - Reuse and Recycling
There are many uses for reused materials in construction, although most typically they are high value finish materials. Some materials are very easy to use, and others require a lot of effort. The biggest difficulty is that many of them require a lot of time to gather and a lot of space to store them. Unlike new materials, reused materials often come in weird sizes, and often are one of a kind (or at least not in the quantify you want), so you typically don't go looking for a specific thing. Rather you find something you like, and try to find a place to put it.
Of course, the best reused material is one that is left in place and just refinished. Assuming that isn't practical, there are three classes of reused materials, each one with its own level of difficulty.
Reclaimed & Re-manufactured
These are products that someone else has reclaimed and is selling in a form ready to use. Common products in this category include re-milled timbers and re-milled flooring. Another example is finger jointed wood, which comes both as 2x4's and in moldings. In general these products are drop in replacements for the equivalent new product.
Salvage
These are products that someone else has removed, but they are typically in the "raw" state and need some cleaning up to be used. You find these materials at salvage yards, and sometimes at antique stores. Example materials include flooring, doors and their hardware, windows, cabinets and their hardware, moldings, high value wood, lighting fixtures and plumbing fixtures. It is usually in pretty good shape, but often tarnished in some way. These materials will need some work to be usable: for example wood flooring might have a stray nail or staple and probably needs to have the excess old finish scraped off its edges. Odd size materials is the norm. Some of the materials, like toilets and windows can't be used as replacements for new one, because they don't perform as well. Old toilets use too much water and old single pane windows are best used only in decorative situations. Finally, when using any old material with paint on it, there is a high chance it contains lead, so you need to sand/cut it with caution.
Trolling
These are materials that no one has processed at all: you find them on your own site or maybe at someone else's. You have to do all the work to remove the material as well as the work to make it useful. This is the hardest thing to do, but also the most fun because it encourages you to be very creative. Finding useful material in your own scrap pile is highly rewarding.
Recycled content
When you can't find anything to re-use, there is always recycled content material. Some standard building materials like MDF already use post-industrial waste, but not many use post-consumer waste. Most iron and steel has some recycled content, and fiberglass insulation is starting to use some also. One good example is cellulose insulation, which is often 100% post-consumer.
One dilemma that often comes up with recycled content products is that they are a composite of materials, and so many of them are difficult to recycle.