@Mike Mariotti - I have used excel for this kind of functioning. For instance, you can simply make tabs with each of your lists.
-Arrange your data so you have similar formatting for each list. Make sure names, addresses, etc. are formatted the same way.
-Add a column for a unique identifier on which TYPE of list (i.e. all tax delinquency properties have a column that labels them as TAX, all driving for dollars properties have a column that labels them DFD, etc).
-Combine your separate lists onto a NEW tab, called Master List or something.
-Sort your Master List by your desired qualifier (address, property owner, whatever).
Now, you can see which properties are 'tagged' under multiple categories.
Assuming you get your data in similar formats, that should be pretty workable (and free if you have excel).
Now, if you're working with a mountain of data, and the formatting is wildly different, this probably won't work for you. But that has not often been the case in my work.