My parents 2000 SSEI has a broken (missing!) pulley, what appears to be the supercharger tensioner. Its kind of in the middle of the motor, near the bottom and uses a torx bit.

Does anyone know the part number, or name of this? The local GM dealer isn't too much help.

Which one?

perfect! looks like #9. is this common at all? never seen that before.

It'* been known to happen from time to time. Sometimes they start to make noise before they go, sometimes it does it silently. It may be time to start replacing them, to prevent future problems. You may want to do all of them in this belt path now, while you have the belt off. That would be 3, 8, and 9.

Hey there, mine did the exact same thing a few weeks ago. The bearing was still in place but the tensioner was nowhere to be found. I ended up replacing all the pulleys at the same time along with a new belt. Cost me about $70 and 30 minutes of work. Just make sure you have a T-50 Torx socket handy!