This page describes the houserules affecting the Ritual Caster feat.
Prerequisite: Trained in Arcana or Religion
Benefit: You can master and perform rituals of your level or lower. See Chapter 10 for information on acquiring, mastering, and performing rituals. Even though some rituals use the Heal skill or the Nature skill, the Arcana skill or the Religion skill is required to understand how to perform rituals.
House Rules add the following text to the feat:
When you level up, every even level you receive one known Ritual of your level or lower for free.
The other variant sounds like this:
When you level up, at 3,6,9,12 etc. (every 3 levels) you receive one known Ritual of your level or lower for free. You can retrain those Rituals later.