Another foundation "product" that I use is the really cheap wrapping paper rolls from the dollar store. Check the labels and buy the one with the most square footage. Bonus using the wrapping paper: there is a printed 1" square grid on the backside of the paper which I have found to be accurate.
Other things I've used as foundations: muslin (makes quilts very heavy), old sheets (most of mine are not 100% cotton so I tend not to use them), interfacing, tissue paper, phone book pages, scribble pad pages and coloring book pages (both dollar store items), but mostly I use copier (computer printer) paper. It is heavier than a lot of the other foundations, but since I stitch with a short stitch length (1.8), the paper removes easily.
Yes, people are correct when they say the foundations stabilize the bias edges.
One last thought: I prefer not to use dryer sheets for two reasons: (1) they are polyester (yes I know interfacing is polyester, too) and (2) I don't like the residual scent (even with unscented ones there seems to be a chemical scent left).