I like really long shirts, but they are hard to find in my size. I pick up men's dress shirts at the thrift store and add darts at the waist. I've keep looking for flannel shirts I can use in some quilts, but, for some reason, they are rather pricey at my local thrift shops.
Last year, I started buying cotton sheets for my quilting at the thrift store. The patterned sheets I have been using as backings, but I've gotten some great plain white sheets I have used for the front of quilts too.
I have a bin full of old denim jeans that have been given to me by friends and family. I've made one quilt and several bags out of it so far, and have used snippets for mending work clothes for my son. I'm also considering using the denim to make some seat cushions and throw pillows.