Katie has a penchant for the plastic drain pipes. We had to change the exposed drain pipes on our down-stairs bathroom sink from plastic to metal because she would chew the plastic piping, and we wouldn't notice until we had soaked our feet trying to wash our hands. It took three times before we had the sense to just change to metal drain pipes. I let her keep a plastic J-trap as a toy, after the switch, and she loves it.
She also loves glasses, remote controls, and pretty much any other plastic.
Edit: oh, and DVD cases, and water bottles are delightful toys too.
-Nicole