I am a big fan of these. I don't consider myself a germaphobe but the idea of E eating directly off the table at some of the restaurants we frequent (like the slightly dodgy Ethiopian place we went to on Friday) grosses me out. So I carry these with me.
They're a good size and have adhesive strips at the top and bottom that allow them to stick to the table pretty well. E is then free to eat his finger foods and I don't have to worry about him knocking a plate to the floor. When he's done I just pull the whole thing up and throw it away on my way out.
Amazon has them in a couple of different package sizes and patterns, but the price usually works out to about 50 cents a placemat.
I saw that Target carries these now.