I am curious in exploring some CRM tools that are alternatives to Salesforce. Ideally, these would all work alongside Marketo as well. What alternatives to Salesforce are out there? Anyone have any specific recommendations or warnings?
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.