I came across this article today about 10 business ideas that sound dumb but appear not to be in retrospect. Your definition of "dumb" notwithstanding it still makes for an interesting read.
My top picks are SantaMail and LaserMonks. My personal favourite is SantaMail, as Hugh MacLeod is wont to say "The market for something to believe in is infinite...". It is a simple concept with clear execution and focuses on building up dreams. Very powerful! :).