Lets face it we all have a can or two of left over paint lying around. Ceiling paint makes a great replacement for gesso and matte white paint. So go ahead and use it it was just collecting dust in the garage anyway.
Save those glass jars from spaghetti sauce or jelly they make great reusable containers for water to wash brushes while your working.
Plastic tops from cans make great mixing pallets or a surface to pour glue onto. They are also reusable. Acrylic paint is easily washed off or peeled off of them.
Makeup sponges: they are cheep and disposable. You can use them to sponge paint or sponge over a stencil. Just make sure to wet them and ring them out well before using them, if they are wet they will suck up less paint.
Magazines and news papers: they can be a great place to find graphics and text to collage with. Go ahead and cut them up, they were just going to the recycling bin.
Tissue paper: If you get a present or package in the mail with tissue paper don't throw it away. Its a great way to add color and texture to collages. Crinkling it up will give you texture. You can layer it with different colors because it's slightly trans parent.
Rubber erasers: Stamps can be expensive so why not make your own stamp. Just draw out a design with a pen and carve it out with an exacto knife. you can use both sides of the eraser and get two different stamps out of one.
Coffee grounds: yep there is a use for left over coffee grounds. They can be used to add an old distressed antique look to paper. What I do is peel up the sides of the used filter with coffee grounds in it so that you have a little round purse of the grounds and I dab the moist grounds onto the paper you can do it to just the edges of a paper to give a faux water damaged look or to the entire paper. the paper will need to dry though before you can work further with it.