Here's the invitation we're sending out to friends and family for our upcoming wedding. She's a journalist, I'm a designer—this seemed to be the way to handle it.
Printed on newsprint, 13"X22" (roughly the size of a normal newspaper), folded, enveloped. Still in production mode for the real thing, but here's the image.
(n.b. The time and date and location have all been changed here, so don't buy a plane ticket or you might be kinda disappointed....)