Oilcloth is a laminated fabric. Totally washable, which makes it perfect for reusable lunch sacks. There are tons of ideas and instructions out there for these, and fabric.com had oilcloth on sale, and I had a coupon, so I thought we'd try it.
I found the easiest pattern from Skip To My Lou. She's got a great site. I used her pattern and added velcro and "embellishments" to personalize them.
Oilcloth is washable, but still plastic, therefore not such a great idea for direct contact with your food. Keep that in mind.
Here's how we did it: