Salesforce Labs has put out a series of Mass Action packages on the AppExchange. There is Mass Transfer Opportunity Owner (uses AJAX), Mass Update Contact Addresses (AJAX), Mass Update Opportunity Owner, Mass Update Opportunity Close Dates, etc. All of these are "thick client" tools, meaning that data must be loaded onto your computer, altered, and then uploaded back to Salesforce.
Ron Hess released a super bit of code, found in the Visualforce documentation and elsewhere, to Mass Update Opportunity Stage and Close Date. It could, theoretically, be expanded to other fields as well. This got me thinking about doing so with other objects.
A while ago, I took the code for the Mass Delete button and altered it to change the owner of the selected records to the User clicking the button. It was like an Accept button, but I could use it for any object. Actually, it wasn't as easy as that. I had to make one button for each object, and maintenance was a pain--whenever I created a new object, I needed to clone my code.
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.
Go to Setup -> Contact -> Buttons and Links
Create a new button, set it to execute in the current window with sidebar and header, and set it to call a Visualforce page. Select the page you just created. Then add the button to Contact Search Layout and to any Contact related lists you like.