Cynthia has an intriguing post over at Chicago Genealogy -- "The Chicago Fire: Was Your Ancestor Insured?" about the possibilities of using insurance records to learn more about your research targets. Interestingly, most of the materials she's found are in the Minnesota Historical Society. (Hat tip to the Newberry Library blog.)
Locally the treasure trove is at the National Archives Great Lakes Region. Bankruptcy cases are federal cases, and most Illinois-based insurers were bankrupted by the Chicago Fire (and not just because it was a big one -- they had been conducting business recklessly as well). So one entry point to insurance matters is through bankruptcy cases in 1871, 1872, and thereabouts.
One of my research targets was in the insurance business, so I had occasion to pay a very pleasant visit to NARA Great Lakes, out on South Pulaski, last summer. (None of what I say below should in any way replace your calling an archivist there before showing up -- they are very helpful, and these records are not simple to deal with. I'm not blowing smoke; check out the on line info on Record Group 21, Records of the U.S. Circuit and District Courts, Northern District of Illinois, Eastern Division, Chicago. Learn from it, but this ain't DIY territory.)
One final repetitive caution: this is not the place to start if all you have is a name and a handful of census lookups. Get to know your people before you start in on this fascinating and rarely-taken research journey -- who they worked and lived with, who they associated with. As Tom Jones says, it's about identities, not names.
The above has to do largely with post-1871 Chicago research, but Martin Tuohy of NARA Great Lakes has a thorough and inspiring article, "Federal Court Records: Researching Hoosier Family History at the National Archives-Great Lakes Region, Chicago, 1817-1859," if you can lay hands on the Spring/Summer 2008 issue of The Hoosier Genealogist: Connections (volume 48 issue 1), published by the Indiana Historical Society.