Each of our classes does either a classroom collaborative art project or a themed basket. Many of the themes we have used have already been mentioned.
One thing we implemented last year that went over really well was "Jump-Start Kits." Basically, we purchased (through donations) the entire supply list for each grade level and put the items in a backpack. We did a boy and girl backpack for each grade. One stop shopping for back to school! They were one of our heavier bid items. Each one sold for well above the actual amount of goods inside. I guess just not having to shop for supplies at back to school time, and having to go from store to store to find everything on the list, is well worth the extra money
We will do this again this year. We will have a coin drive to purchase the supplies. Each winning classroom from each grade level will win an ice cream party for their efforts. It's more fun for the kids than just hitting mom and dad up for money.
As for class collaborative projects, we have a teacher that does a quilt each year that incorporates the students artwork. I did a project that works well with lower grades last year that involves watercolor hand outlines. (If you want more info, PM me). Handpainted table runners; collage painting; tiled mirror (each child makes a tile-a parent finishes the project); tiled outdoor table; thumbprint drawings; handprint drawings; etc.
Obviously the classroom project depends on the age level and parent/teacher involvement. Most classes choose to do a themed basket.