We've all been there - completed a project with a partial skein leftover that had no purpose. There's too much to simply throw away, and you don't want to be wasteful. Try this trick I learned a long time ago: ball it.
Take your first partial skein of a particular weight and fiber and wind it into a ball. Now, every time you finish a project using the same weight and fiber add it to your ball. Eventually you will have enough there to make something with.
It is very important that you keep the fiber content and weight of the yarn consistent in your ball. You don't want to finish an item, only to have part of it shrink or felt or otherwise get ruined in the wash.
Worsted weight acrylics and acrylic blends make wonderful scrap yarn afghans. A friend calls them, "so ugly they are beautiful". Usually these are on a bed, so they will not be display pieces, but still warm and functional.
Worsted weight cottons can be used to make unique handbags and tote bags. Many commercial versions of these products are of various color and print, so your crochet version is not likely to be considered an eye sore.
Worsted weight and sport weight wools can create wonderfully eclectic sweaters or set the tone of a room as a rug. Baby yarns tend to have the same color tone, so you can create more baby items with them. The possibilities are as endless as your imagination!
In the long run you will be saving money as well. Once, when I had just started a new job and had nothing extra to purchase yarn with and Christmas fast approaching, I made a rainbow afghan entirely out of scrap. I had a bin full of partial skeins of worsted weight acrylic. I took each one, balled it, and then lined them up on the table in the order of a rainbow. Various shades of the same color were put in order of what I thought looked nice. Starting with the purples (violet is the last shade of the rainbow) I rolled a ball in reverse order - violets, indigo, blues, greens, yellows, oranges, and finally reds. Upon crocheting it in rows of double crochet, the color effect was simply stunning. Small amounts of one color and larger amounts of another, added with the various shades of each, really did mimic that of a rainbow. I don't think I would have put all of those colors together if I had gone out and purchased the yarn. I had made an absolutely beautiful, one of a kind afghan without spending a cent AND I cleaned up my yarn closet in the process!
Give it a try, and share what you come up with!