Southeastern Wisconsin Archives All In One Place?

Well, no, not really. I try not to make a habit of recommending 13-year-old internet resources, but I like the philosophy behind the Online Guide to Genealogical Collections in the Milwaukee Metropolitan Area, produced by the Library Council of Southeastern Wisconsin, Inc., and hosted by the Archives at the Golda Meir Library, University of Wisconsin-Milwaukee.

For one thing, they define their area broadly, including Milwaukee, Ozaukee, Sheboygan, Washington, and Waukesha Counties. And they made the best of it when it became clear that they couldn't list everything available in all those counties: the guide focuses on official records of churches and governmental bodies, and within those

Many records exist which can be of great use to genealogists, but are rarely examined. The Guide contains entires for many of these kinds of records, especially those of civil and criminal courts, fire and police departments, and personnel records...

Just in places where I have research targets, I found delinquent tax lists 1862-1926 for Sheboygan County and assessments for the Town of Plymouth 1861-1877 (at the Sheboygan County Historical Research Center), Kenosha County tax rolls 1856-1965 and assorted church and cemetery records 1839-1952 (at the University of Wisconsin Parkside), and Milwaukee County school censuses 1913-1960 (at the Milwaukee County Historical Society).

The lists are not consistently alphabetized, i.e. Kenosha church records are under "C" not "K" or any of the denominations. But in any case this is a source of inspiration for when you're stuck, and a metaresource: in all cases be sure to check the web site of the alleged repository for up-to-date holdings information before you grab your laptop and head out!