Paper towels. Use rags cut from old clothing, linens or towels. Cut and hem them and they will last a very long time, wash after wash. You will no doubt wind up with clothing or other material that isn't absorbent or won't wash well. Cut those up and save them for nasty jobs so you can throw them away like you would paper towels.
Paper napkins. A very simple solution is to cut some squares of cotton or other absorbent material and hem it. Voila! Instant cloth napkins. Some people buy washcloths in special colors to use as napkins, but making your own is cheaper; your choice. Wash them with the other kitchen items like dishcloths and teatowels. Also, it doesn't hurt to use the same one a couple or three times if everyone has their own color or design (unless it's a messy meal!)
Trash bags. For the smaller trash cans, use plastic grocery bags. They're a sturdy or sturdier than bags you buy and you have them already! They even have handles. For larger trash cans, save bags that dog food, bird seed, mulch, compost or other garden amendments come in. Anything that comes in a large enough bag works. I have used the large plastic bags from Goodwill or other thrift stores (or even retail stores!).
Pots for starting seeds. Use any small container that you can poke a hole in for drainage. Some people use egg cartons, putting soil and a seed into each egg compartments. This is enough to get started, but plants will need to be transplanted soon because there is little room for roots to grow. Think: Soft drink bottles, cut down, yogurt containers, cottage cheese and sour cream containers or any plastic container that you can cut down to size.
Bulletin boards. Need a place to pin up pictures, notes or lists? Get a piece of cardboard from a cardboard box that is the right size for your space and cover it with piece of solid color cloth. Better if the cloth comes from an old sheet or tablecloth that you were going to use for rags! Wrap the cloth around the cardboard and tape it down with duct tape. You can either poke holes in it to hang from or attach a string from each corner to hang it. A few pins or thumbtacks and you're in business.