1. Rotor alignment tool: With advances in MTB disc brake designs and technology, brake systems are becoming more powerful and reliable, but pad alignment and noise is still an issue that plagues with most systems, especially as pads wear and rotors warp with use. A rotor alignment tool like this one can make it easy to quickly adjust your brake pad alignment on the trail. You could do the same thing with DIY tools, but this one is quick to apply and easily packed with the rest of your trail tool kit. Less time fixing, more time playing.
2. A small roll of duct tape: I carry a 1-inch roll of Gorilla Tape in my hydration pack, which is a small roll that doesn’t take up much space. Useful in many situations, from a temporary fix of a small tire sidewall tear to securing a broken spoke.
3. A universal derailleur hanger: you should carry a frame-specific hanger anyway, but this a universal derailleur hanger will be great if one of your ride buddies (who isn’t carrying one) breaks his and you’re far away from the trail head.