There is none, at least directly.
You can create a lead performance report and group them by company name. The number of rows in the report will give an indication (not very accurate, as they might be some homonyms and some weird values, gathered from forms.
If you use SFDC or Dynamics, you can add a smart list to select CRM contacts only, it will give an information that will be accurate, but you might as well get it from your CRM.
An API was recently released that gives access to the company object : Marketo REST APIs: Company Objects » Marketo Developers
I ended up doing an export to Excel and then removing duplicates from my Company column.