Minecraft wiki entry for Quasi-connectivity, associated text: placing or destroying a block next to a piston
moving a block next to a piston
changing the state of some blocks next to a piston (for example, changing the delay on a repeater)
changing some states of some redstone components within two spaces of a piston:
changing the state of a redstone torch within two spaces of a piston
changing the power level (but not the orientation) of redstone dust within two spaces of a piston
changing the power level (but not the delay) of a repeater facing a space next to a piston
changing the power level or changing the operating mode of a comparator facing a space next to a piston