Back List maintenance software

I am looking for a software program that can take a list of prospects ( including name and address) compare it to another list of prospects or a list of clients and remove the duplicates. Does anyone have experience with a program that works well for them?

You can do this in Excel or in Access.

@RickAnderson for best dedup of mailing addresses, you'll need to standardize the address data.
Qualified Address http://qualifiedaddress.com/ is a simple service that will validate, standardize and dedup your address data. It's simple as pay, upload your file and get back a cleaned list. Fees are based on size of your list.