At 15 they should be starting to prepare for their independent life, so really they should have all relevant responsibilities but in a controlled manner. The obvious would apply (i.e. cleaning room, doing homework and maintaining grades etc) but you could expand by being in charge of planning and cooking dinner one night every week, doing their own laundry, paying their own cell phone bill etc. Basically anything they will need to do when they eventually don't have mom and dad around to do it for them or remind them! I've known people who have had everything done for them growing up and then get totally overwhelmed when they move out because they've never used a washing machine or oven before!
If you are wanting to have a cell phone, car, expensive brand name clothing, going out on the weekends, etc.... You could get a part time job and help with the expenses that go along with all of those.
I disagree with washing the car! Wash your own car! But I do agree with doing household chores (which everyone should help with). Most important is focusing on school because when you graduate, everyone is going to look at your grades and gpa before accepting you into the school or giving you scholarships