Drawing upon some code I found in the Developers Guide, I have written a Visualforce page to mass-change the owner of all selected records to any active User in Salesforce. Yes, it only works with Contacts for now, but my next step will be to use Dynamic Apex to pass the sObject name to the page and its extension/controller, allowing me to reuse one bit of code for multiple objects. We'll see how it goes. But in the meantime, enjoy this page that lets you perform a search, check which records you want to transfer, and then input a User. I've tested it with 50 records at once, so that should suffice for most uses.