Todd Karr and Richard Hatch, in the Erdnase thread, have presented some of their research which came from old newspapers. I've been recently helping Mark Damon with some material on Johnny Oaks that I've found in newspaper databases. Digitized newspaper archives are a trove of information, largely untapped by magic researchers.
There are four big online digitized subscription archives.
1. Newspaperarchive.com HERE -- Lots of newspapers, all over the country, but what they have almost seems random. I think they are associated with Heritage Microfilms, which does most of the U.S. microfilming of newspapers that you see in public and university libraries, and are slowly digitizing the microfilms that Heritage has done. You can subscribe directly with them ($71.40 per year) or by joining Ancestry.com ($179.40 per year; more expensive, but includes access to other data sets and archives).
2. ProQuest.com Historical Newspapers. HERE Supplies digitized searches for:
New York Times: 1851-2003,
Wall Street Journal: 1889-1989,
Washington Post: 1877-1990,
Christian Science Monitor: 1908-1993,
Los Angeles Times: 1881-1985,
Chicago Tribune: 1849-1985,
Atlanta Constitution: 1868-1929,
Boston Globe: 1872-1923,
Hartford Courant: 1764-1984
Normally, you access these through public and academic libraries -- they don't really market to individuals, like Newspaperarchive.com does. However, up until recently, the Society for American Baseball Research (SABR) has had a group subscription that let you get all of the above except the Christian Science Monitor, the Wall Street Journal, and the Hartford Courant, but it added access to the Chicago Defender 1905 - 1975 (a major African American newspaper) and the digitized version of the American Periodical Series of microfilmed magazines and periodicals. SABR was a good deal -- $60.00 -- but they just announced that ProQuest is withdrawing the package that they used, and won't supply it beyond 2006. I haven't found a good way to replace it. Check your local public and university libraries; they may have on-site (but probably not web-based) access, for free.
3. Paperofrecord.com See what they have digitized online HERE :
They cost $100 per year, but you can get a discount by subscribing through SABR. They are somewhat less useful than the services above -- their search interface SUCKS. You can only search one title at a time, and only in five year increments. So if you want to search 100 years of The Sporting News, it will take 20 separate searches.
4. Early American Newspapers (title list HERE from Readex/Newsbank. I don't have much experience with this one, but it is slow, and the OCR isn't too hot.
There are also a number of limited, free newspaper searchable archives:
The Brooklyn Eagle HERE
Utah Digital Newspapers HERE
Colorado Digital Newspapers HERE
Missouri Historical Newspapers HERE
Northern New York Newspapers HERE
And some subscription backfiles:
Dallas Morning News (through Newsbank.com)
London Times (through Infotrac)
Toronto Globe and Mail (supplied by ProQuest Canada, but uses Paper of Record's search interface)
And there are a number of more limited, specialized searchable fulltext databases that don't have as much use for magical research -- university student newspapers, for example (the MIT Tech and the Harvard Crimson files are online).