Sales Reps in a B2B world need quite some time to close opportunities in Salesforce, so much so, that they need to log sales call notes constantly and follow up on many activities for each of their deals. Many of our customers are also using activity logging apps such as "Logger" that allows a lot of activities to be logged from the iPhone / mobile device. As a sales representative or sales manager, it becomes very difficult to know where a rep needs to pick up on an opportunity where he/she last left it.
First, create a new custom field (using Text Area (Long) as the data type) on the object you want the results to reside, in this case Opportunity. The name of the field can be "Call Notes."
Create the rollup: