Emergency blanket -- Of course, the blanket's main purpose is helping you or someone else stay warm. It's especially useful in a first-aid kit if someone is going into shock. Wrap the blanket around the person, tucking it in on the sides and under the feet in order to keep body heat in and cold out. Cover the person's head with a hat or scarf to prevent further heat loss.
Emergency shelter -- In a pinch, you can use your G.I. blanket as a tent, tarp or lean-to. The material is waterproof so it will protect you from rain or wet snow. If you are in a cold weather situation, take advantage of the shiny side of the blanket to reflect heat from a campfire back to you. The metallic surface will bounce the heat of the fire toward you and help keep you warm.
Keep warm -- Besides using a blanket as a blanket, you can also use it to insulate your space. For example, let's say you become trapped in your car during a freak winter blizzard. Cover the windows of the car with the space blanket -- shiny side in. It will help reflect your body heat back inside the car.
Keep cool -- In the same way that the blanket reflects heat back toward you, it can also reflect heat away from you. If you're in a tent, and the sun is bearing down on you, put the blanket shiny-side-up on top of the tent. Have you ever seen shiny sun shields on the windows of cars during the summer? Same idea -- the metallic surface of the sun shield reflects the heat of the sun out of the car.