Make:

We all need materials to make things with, right? Well here is a basic list of a lot of the materials that might be good for making or crafting projects. This list is not intended to be a final list, but rather a starter list. Sources for purchasing these materials were intentionally left out, because I think that most everything on the list can be pulled out of the waste stream and reused. If you have to buy it, do so sparingly so you can save up for the things you might actually need to purchase. If you see something missing, just add it into the comments. You might even see a material on the list that you haven't heard of or used before. Here it goes!

organic/pulp based
Tissue paper - Thin, not very structural, it can be taped or glued. It comes in lots of colors, and you can get it by saving up from all those bridal and baby showers you get invited to.
Newspaper - Every day another one shows up. Made of wood pulp and covered with ink, if you roll it, you can make tubes, or it could be folded or torn, or...
Printer paper - If you need a decent piece of paper, there is usually at least some of this stuff around. Maybe you have to go into the recycling bin looking for old memos, but there is more to be done with it than paper airplanes. You can also get designs that you print that have the fold marks right on them.
Magazine pages - The covers are heavier than the pages. Nice pictures, glossy paper.
File folders - Sometimes you can find them around with nice colors. The material is stiff, easily cut and can be taped, stapled or glued. Tab and slot designs work well.
Paperboard - Cereal boxes are a great source for this stiff, light cardboard. It seems like it is usually made of recycled paper and is gray or brown, short fibers.
Milk cartons - After you finish your cereal, save the milk carton, because you can make lots of great stuff with the water resistant cardboard that carried your cow juice.
Corrugated cardboard - Boxes, appliances, everything from amazon or ebay provide a ready supply.
Balsa - Light, soft and structural. Balsa gets used for lots of model making projects. Many model airplanes have been made of balsa, but the highest balsa adventure of all time has got to be Kon Tiki.

Plastics
Report covers - Going to a conference? Bring back some nice clear stuff. It measures about 11" x 17" when a report cover is flattened out.
Plastic sheet - This comes in a variety of thicknesses. Cheap plastic dropcloths, and all the way up to thick black landscape plastic or more.
Plastic jugs - Milk, water or apple cider jugs can be cut with a utility knife.
Salad containers - Use these clear containers to form clear sections of your constructions.
House wrap - Usually made of polyethelyne fiber, it comes in rolls for providing a vapor barrier before the siding goes on your house, this cuts nicely with scissors or a utility knife.
Boat wrap - When your neighbor sets sail in the spring, grab some of the white plastic they used to cover the boat you had to look at all winter. It is neat stuff, and is designed to shrink under heat. Nice and heavy duty, you can actually make yourself a boat of it.
Coroplast - You might know of a store going out of business that has just splashed the news all over town with their "Going....Going....Gone...." signs. When they close the doors, what are they going to do with all those signs? Coroplast is great stuff, structured like cardboard, but weather resistant. It can be scored, cut with a knife, on the bandsaw or with other cutters. You can fasten it with zip ties or tape it, or fold it like a cardboard box. Lots of potential here.
Soda bottles - Cut the top and bottom, slit the side and you have a decent piece of plastic. Suitable for making safety glasses with.

Foam
Take out trays and meat trays - If you cut off the curved parts, there will be some nice flat parts to make things with. You could try a hovercraft, but there are other things to be done after you finish the leftovers.
Packing foam - Some things like picture frames come packed with sheets of white foam. Usually, the packed stuff is made of lots of little foam balls stuck together. It is kind of messy to work with, but has a decent R value, and is kind of rigid.
Foam core board - Cut it with a sharp utility knife, glue it on the edge, paint it, spray adhesive coverings on it, nice to make architectural models from. You can probably find it in leftovers from presentations and science fairs.
Sheet insulation - This stuff is really fascinating. It is sold in building supply stores as an insulation material. Scraps can probably be found near newly constructed buildings. It usually comes in pink or blue and a range of thicknesses. It can be glued with wood or white glue, screwed together, drill it, cut it by scoring with a utility knife, table saw, jigsaw, bandsaw or with a hot wire if you want a nice clean edge. This is a great starter material for CNC tools like the shopbot or mill. It gives very little resistance to the tools, allowing you to build the technique and process before going to more expensive and less abundant materials.

Metals
Aluminum foil - It's in your kitchen, comes off your sandwich, conducts electricity, bends, folds and goes into the recycling bin when you are done.
Pie pan, turkey pan - Thicker and holds its' shape better than foil, pans can be cut with scissors, and if you want to try your hand at boatbuilding, you can even make a press fit mold for the hull.
Aluminum can - This can be cut with scissors once you get it started by piercing a can with a utility knife. Now that those huge, overly caffienated drinks are all the rage, cans can be turned into nice decent sections of aluminum. Sometimes you can incorporate the graphics into your design.
Tin can - Harder to cut, you will want to use aviators' snips to get this flat. It can be fastened with screws.
Flashing - Building suppliers sell flashing made of aluminum and copper. The copper is expensive, but really nice, and can be soldered. Aluminum comes in a zillion configurations, rolls, rectangles and more. You can also get larger sections of it as well, solid, or punched with holes and patterns.
Computer cases - Are you finally done with that 386? After you scrap out all the components, you will find some wonderful sheet steel. Usually the outside is beige, and the inside has a clear coat on it. 1/16 inch is pretty common. Fasten it with sheet metal screws or rivets. you can paint it or leave it beige. There is much to be done with steel.

So there is a semicomplete list of the materials you might want to be on the lookout for. What are your favorite materials to work with? Where do you get them? What is the best free source for materials? How much of these materials can you scrounge out of the recycling bucket? Post your suggestions in the comments and lets share the storage bin!