I'd second Scribus, but that's probably more than you need -- it's intended for desktop publishing. If you're doing a simple mailer, I'm assuming you're talking something more like a post-card, than a multi-page brochure. If that's true, then Tony's right -- you probably have all the functionality you need in a word-processor. If you're doing something a bit more artistic, then you could give Inkscape a try -- I've used it quite successfully for a number of brochures, postcards, and other print jobs.

The company my wife uses for all her theatre printing is Overnight Prints. They have templates for all of their products, that you can download, including ones for Adobe Illustrator, which import into Inkscape just fine. They accept PDF, as well as plain old images. I'm pretty happy with the quality from them.