What has actually been used is a good addition to this thread:
spare fin strap - someone diving near us broke a strap getting out of the water, I gave them one of my spares; bandages/gloves/tape - nasty cut from a training platform; multitool; adjustable wrenches; o-rings; ear beer gets used every dive; zip ties; battery tool/battery; silicone gel for camera o-ring seal (forgot to mention that); almost needed to use spare mask - wife dropped hers, but a student doing AOW nearby was given the task of finding it for his search and recovery dive!