A mandoline food slicer will make flat uniform slices, anywhere from paper thin to 1/4-inch thick (or more). My basic model even has a Julienne insert for making French fries. There are fancier ones available that do wavy slices and zesting too.
I used mine for cucumbers, onions, sweet potatoes, zucchini, and other vegetables. It can also be use to cut fruits and cheeses.
Here are some of my favorite recipes made easier with a mandoline slicer: