Last school year I started a co-op for preschool. It consisted of 6 families. We chose a curriculum to follow (The Complete Daily Curriculum for Early Childhood and the letter books from http://www.first-school.ws/) and then traded off the home to have it at each week. We met twice a week and a helper mom would stay to help with crowd control.
This year we are doing somewhat the same thing. We are integrating a workbook and phonics and using the activities not used last year in the lesson plans. We are also not having the helper mom stay at the home, but she will help with the younger children. Yes, this is mainly for me. Today I taught for one of the sick moms and my helper mom took all three of the little girls to her house. This was quite nice of her. She has one little girl at home who is 1 yo. Her and Katia played together while Emma and Delta were bounced.
My main reason for starting a co-op was to help Jayden get out of his comfort zone and go to other places without me. I was quite worried this year as Jeanette won't be there any of the times. But he did great the first day and even let another mom help him get to the house so I didn't have to take him the whole way. I was excited.
The other reason was the cost. Other than supplies needed, it is free. It costs time and art supplies. This is a great deal compared to $60-$85 a month.
I have enjoyed this and it has helped me not miss teaching school the last 2 start of school seasons. Maybe it's because I get to use the new crayons.