In terms of powerful cleansing herbs, rosemary is my second favourite and one of the most effective (just after rue plant, but rue plant is simply the best).
It's one of the most basic herbs that can be used and I highly recommend it.
To grasp the nature of this plant and in what kind of rites we can use it I think we should start with its etymological analysis; in Spanish its name is Romero, a word that applies to a person who goes on a romería (both could be roughly translated as pilgrim/pilgrimage although this words don't convey the truest meaning). A romería is a kind of travel, usually lasting somewhere in between one week and one month, done with religious purpose. In the south the most common romerías are those in which the statue of a virgin is carried from one village to the nearest one, signaling the start of the summer festivals. Two examples would be El Rocío and Santiago.
Nowadays this has lost all its mystical sense and it's only done for partying, but in origin the road was meant to cleanse yourself, to get rid of toxic psychological habits and throw out the negativity from the lives of those who chose to go on the road as pilgrim/romeros, and THAT is what we can achieve by using rosemary in our rituals, to bring the spirit of the pilgrimage, one of soul and not of body.
Here are some ideas of ways to use it:
-Boil it in water and add the result (or essential oil, but I always prefer to use the fresh herbs) to the bucket of water for cleaning the floors of your home. Start from the room farthest to the entrance door and swipe towards outside. After this throw the water in the street/outside (never in the toilet or something like that). This is the most effective way of cleansing for a house.
-Put one strand in a bowl of water under the full moon, this way it can be used to cleanse and charge stones and other utensils (this can be used as substitute of water with salt for the stones and metals that can be affected by the salt). For more efficiency on charging stuff under the moonlight instead of putting things just on the night of full moon do it when it's third quarter and leave it until the full moon. (NEVER put stones and/or whatever you want to charge under a new moon, the new moon energy 'takes away', it doesn't 'give')
-Essential oil can be used to clean anything, from any surface to even yourself.
-It can be used for cleansing also by burning it and passing stones, jewelry, yourself, etc, through the smoke. This I recommend mostly for persons, as opposed to the essential oils that I like to use more to clean the surfaces of the house, I feel that by using its fumes is easier to clean the 'non visible, non physical' part of ourselves.
-It can also be burnt alongside a note in spells used to get rid of our bad traits or negativity in our lives. In general I like to use fire in this kind of rites, I'd usually write down the spell and burn it while reciting. By adding rosemary to the fire we give a boost of power to it.
-Hanging it by the front door of your house and/or by the windows will protect your home. There are various plants that can be used for this and rosemary, as said before, is one of the most effective. It's even better if you can grow a plant yourself in your yard (and it will smell heavenly on summer nights).
-In spells where you want to write down the effect in a candle so it burns away slowly you can also use a strand of rosemary to do it.
-It can be kept alongside your tarot deck.
-It's awesome for seasoning meat, and when thrown in the fire of a barbeque the smell would give the meat a great flavour.