This functionality is not available out of the box on a summary report. However, this is a workaround to get this to work on a summary report, although it technically isn't ideal. To do this, you will need to do the following.
First, add a new custom field to an account called Count. This field should be a formula for a Number with a value of 1, like such:
This field will be used as part of your report. Make sure that you have properly added this newly created custom field to the report type. If you do not add it there, it will not show up when you go to create the report.
Next, on your custom report, you will need to add a formula field. This formula field will be the sum of all Account Counts and will be displayed at the bottom of the page only. To do this, drag the Add Formula field under the Formulas folder on the left hand side of the report onto the report. Once you do that, you should get a popup to enter values. Those values should be entered as such:
Save the field. Run the report. The result should look something like:
Notice how the new column is only displayed at the Grand Totals section of the report. Also notice how it only sums up the accounts that are displayed, correctly showing a count of two accounts while there are actually three opportunity records in the report. Unfortunately, there is the added column on the right hand side of the report and the count at the bottom of the page doesn't have a label. There isn't really anything you can do about that right now. The good news is that you can now see the Account Count without exporting to Excel.
Please take some time to visit the Ideas site and vote for this functionality to be added to Salesforce as part of the base functionality.