I have this adapter:
Is it completely useless? I'm not quite sure what it is. I've had it for years.
This is a Firewire 400 to iPod 30-pin adapter.
If you have an older iPod, you may be able to sync and/or charge it via Firewire. This site has an exhaustive list: Which iPods can sync and charge via USB? Which iPods can sync and charge via Firewire?
It looks like some models supported Firewire charging as recently as 2009, but no models since 2006 or so have the ability to sync media over Firewire.
Many Apple portable devices used to support charging and syncing over FireWire. It was actually quite nice, because FireWire charged iPods significantly quicker than USB can. I believe these are the devices that support FireWire charging and/or syncing: