MS Office Pro. The stuff ther interfaces with each other and QB or QB Pro can interface with this stuff as well.
You can export customer info to Outlook. MS Word can access your Outlook address book.
I'm just now switching acctg to QB Pro and learning a lot of new capabilities with stuff I already have, like MS Office Pro. I've used it for years but but mostly Excel & Word and about all I know is how to put the spreadsheet info into a word document.