I'm in the process of building a not-for-profit site that will aggregate current and historical property tax data for all counties in the United States and serve it via a REST API. A problem I've encountered is I can't find legal guidance on who owns this data, and if its legal for me to do this. What resources could I use to find ownership information for the data I'm looking to aggregate?
In the USA there are no sui generis/fruits of labour provisons protecting data. While compilations of data may be protetected if the author has used creativity with respect to which facts to include, in what order to place them, and how to arrange the collected data so that they may be used effectively by readers (re: Feist vs. Rural telephone), this (very mariginal) copyright protection afforded collections of data in the USA will not be relevant when you create a new collection reusing current and historical property tax data.
In addition, most states in the USA regard works (which may be collections of data) that has been compiled by the agencies of government or its subdivisions, the property of the people. For detail, see Wikipedia about copyright of US state governments. This is probably not relevant to you, as transformatory use of state tax data is not under any circumstance breech of copyright in the USA, but gives you extra assurance that it is legal for you to reuse and aggregate this data for the purposes you describe.
But to make sure (never rely on legal advice on the Internet), you may contact the agencies of government or its subdivisions that releases the data you plan to aggregate, and ask if they claim ownership to it.
It depends on the data that you are aggregating and what licensing has been applied to it. For example, if you are finding the data on the U.S. national open data portal, Data.gov, there is no licensing applied for data provided by federal agencies and it is open to use and reuse, including for commercial use.