I'm sure I don't need to mention this, but sometimes the most painfully obvious gets overlooked.
Garbage bag ponchos. Pretty self expanitory. Use the heavy duty large leaf bags if you can. They are field expidient, inexpensive in quantity, easy to make, and work pretty darn good.
You can also put one over the end of a sleeping bag if there isn't enough cover wherever a person is sleeping, etc... the uses are endless and limited only by imagination. I keep several in my emergency kit.