I've never had cause to do this kind of research before, but thanks to Ancestry's recent massive update to its Immigration Collection, I found a reference to a passport number for one of my great-grandfathers. He applied for one in 1929 for a trip back to Ireland.
I've had cause to deal with passport applications before -- and have even written about them (see the April 2004 US Passport Applications article I wrote for Family Chronicle here, downloadable in .pdf format). But this was the first time I was confronted with the question of how to obtain a more recent passport application.
So I did some surfing. And here's what I came up with -- instructions from the U.S. Department of State. How well does it work? Well, stay tuned. I just popped a request for my great-grandfather's application in the mail today. Let's see how long it takes and what I get back. I'm hoping for a photo since I've never seen what this fellow looks like!
If I like what I get, I might even request copies of some of my own passport applications, but at $60 a request, I think I'll take my time!