When it comes to saving weight, finding peices of gear that serve more than one purpose can be a great strategy, allowing you to eliminate extra gear you have to carry. What's your favorite piece of gear that serves more than one purpose (i.e Duct Tape for gear repair as well as preventing blisters)? Bonus points for one piece of gear with 3+ different uses.
My phone is my favorite piece of gear that has multiple uses. It's a book, a spare flashlight, a GPS, it has maps, guides, a camera (I have a wide angle lens for it too), a music player (with headphones - don't play music out loud, other trail users and wildlife don't like that!). Depending on the network situation it can also be used as an actual phone.
Believe it or not, I have found several uses for the BearVault BV450 Food Container. In addition to using it to store my food and other items that might attract animals (from bear to mice), I have used it as a stool to sit on, a basin for collecting water to filter, and a wash/rinse tub for washing and rinsing sweaty, dirty trail clothes. I love being able to see what is in it, even if the item I am looking for is at the very bottom. While not as soft as a foam roller, I can also use it to lie back on and roll out my back at the end of the day.
Bandana! You can use it for multiple first aid situations (Tying a splint, bandage, compression, soak it in water to cool someone down), filtering water (you need other stuff to make it drinkable, but it will get some sediment out), Neck or head covering to keep skin out of the sun, and more! That's why boyscouts have them
Crocs. They are great seat covers and even block pointy rocks and jabbing branch ends. They also can be a pillow in warm weather when bringing a fleece doesn't make sense. I've used them as elbow rest for reading on a hard surface as well. I greatly prefer them to flip-flops. I would ford some streams in crocs that I never would in flip-flops. Nothing with serious current but knee-high water for sure. Plus, they may take up more space but they weigh about the same.