I've been asked this question a lot by prospects I'm discussing inbound marketing with. Since I talk to them about how much more effective inbound marketing is compared to traditional print or outdoor (for example) they naturally start to talk about ROI.
My very first job was mowing grass. I had a half-dozen clients in my neighborhood and I started mowing their lawns when I was about 11 or 12. I even had an employee for a short time. I paid for many tanks of gas, loads of casette tapes (Google it) and even a few trips to the beach with money from mowing lawns.
My first real jobs where I had a boss and got a pay check was as a busboy at County Line Barbecue (on the lake) and Comet Cleaners (on Far West Blvd) both in Austin, Texas.