All kinds of things. For years as a business owner I used FOIAs to gain some insight into government contracting work. It was expensive but I think worth it. Lately I've been asking for information about military operations and projects that I know have happened or exist, but that no one knows about (this is very, very hard to do, btw, and I don't recommend it since you'll probably just waste your time). When I hear about classified projects -- for example from Snowden's release -- I FOIA for information that might be tangential but unclassified. That's how you find bombshells, btw. Most of the time these days, I get the "cannot confirm or deny" replies, or "we could not find material responsive to your request". I have some very cool FOIA responses from from the NSA from the six months before Snowden dropped his bomb. "We can't confirm or deny ... National security" and the like. I'm not sure how to say this, but due to my network, I had some insight into what was happening. Don't get the wrong idea about me, but anyway, those replies (one of them dated on "the apocalypse" of 21 December 2012) are fairly frame-able. As I said above, FOIA'ing as a commercial requester was expensive. As a private person asking for noncommercial reasons, you can waive a lot of the fees.
Overall I think the FOIA is a great utility. Most big name federal agencies actually have FOIA liaisons that will try to help if they can. Some, of course, are just as douche-y as you would imagine (e.g. NSA, NRO, CIA). Though I did have a few good interactions with CIA FOIAs, mostly bad ones though. The FOIA actually says in the law that the reviewing official should look at a FOIA as "what can we release?" and not as "what can we hide?", and you need to tell people this more often than you would like. I actually used a FOIA to get my DNA information from the military's secret-but-not-secret DNA database. It's a database of every service member's DNA, and ostensibly is for identifying remains. That was actually very difficult, because nobody had ever FOIA'ed the Army's "funerary affairs" office for DNA information, and so they were under the impression they could just ignore my requests. That taught me a lot and was actually fun in a weird way. You just have to persist, and escalate if you don't agree with the response or don't get any. FOIAs are serious business, and most people in the US government are people just like you and me, and want to help you find what you're looking for. Though, some people in government -- like any other community -- are bullies on a power trip. Wow, sorry for the wall of text!
