Demand Tools
Cool software for managing Salesforce data that is donated to nonprofits
Demand Tools is awesome. Get it here.
The license is free to nonprofits, but each client you use it with must sign up. You don't need a license to download or run the software, but when you log to an account for the first time, you'll get a message that you've started a free trial. You should then ask our customer contact to request a free license from the DemandTools rep. Some features are disabled until you do.
Demand Tools is great for mass updates to data. Its specialty, however, is deduplicating -- for example, merging lots of duplicate contacts according to certain rules and preserving all the existing information and child records.