This a giant question.
First, why do you want to do this? I would avoid having more than one CRM.
Second, how do you want this to work with (or not with) Marketo?
The reality is Marketo has a tight integration with SFDC and MSD, and from there you use the API.
Like Josh mentioned, the two CRM platforms where Marketo has the strongest/native integration is with SFDC and MS Dynamics (SFDC being at the top). While the SFDC integration gives you the best compatibility/features, Marketo is continuously improving its integration with MSD (and making sure the feature set (e.g., Marketo Sales Insight) is common between the two) - given that more and more of its customers are using MSD; and due to the strategic partnership with Microsoft.