I love to re-purpose household items when model making, there is something very satisfying about recycling household waste.
Much like the previous poster, I can also attest that the pantry is a treasure trove when it comes to modelling. My most recent project was this diorama the dead leaves under the trees are actually dried herbs. I bought a large pot of 'mixed herbs' for 89 pence from Home Bargains (UK store) which will last me for years, and most people will typically already have these or similar in their cupboards. You can even pull off some nice weathering effects using your spice rack, coarse salt for example can be used to create nice rust and chipped paint effects, tea can be used to make things look aged. The only downside to raiding the kitchen for your projects is that your models will smell amazing!
EDIT: First picture was shrunk down for some reason, you can see the leaf litter in better detail here:
https://i.imgur.com/n4Nxap5.png
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Other materials I try to keep hold of include;
Old Paper Egg Boxes- Coffee Shop cup holder trays also work as they are made from the same material, these typically have a rough texture and can be used to make really convincing stonework for walls and floors, check out this tutorial someone posted using them for paving stones:
https://www.youtube.com/watch?v=2yMd0s_vbl4
Aluminium Cans - I make my own corrugated metal panels out of old cans using an old metal 'Toothpaste squeezer', which are great for urban or futuristic scenery. Whilst the squeezer isn't technically free, they can be picked up relatively cheaply on ebay and will allow you to endlessly recycle old cans into terrain building materials. The below example is not my picture (I don't have my camera to hand), but was made using the same technique:
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Youtube is a great source of inspiration when it comes to re-purposing household objects for scenery pieces, just do a search for 'Terrain from Trash' and you will find a wealth of amazing modelling projects that were completed using old household junk.