Zip Ties
These can be almost as useful as duct tape. Zip ties come in a wide range of sizes, though short ones can be combined to create longer ones if necessary. They are cheap and easy to toss into almost any size kit. Standard zip ties must be cut off, of course, when their work is done. Reusable ones do exist but they are a little pricier than the standard models.
They can come in rather handy when building a debris hut or other type of expedient shelter. They can also be used to lash items to your pack or to keep cordage from getting tangled. Zip ties can even help keep your pants up. Run one between the belt loops on either side of the button on the front and slowly tighten until the waist is snug.