Tenser's Circular Shield: The item says it's an optional focus for the Tenser's Floating Disk ritual. Does that mean one can perform the ritual free of component costs (10gp)?
Yes, that's correct. An article in Dragon #385, "Ritually Speaking: Item Focus Rituals," introduced the concept of item focus rituals. These allow specific magic items to serve as a focus for the ritual without the need for ritual components. It's a great way to encourage your players to use rituals and to make magic items seem more wondrous.