Are there any Apps or tools that can be integrated with MSCRM

Hi,

I am looking for apps or tools that can be integrated with MSCRM 2015 to ease its different functionalities like sales, marketing, pricelists, etc.,

Currently we are using 'mailchimp' within CRM for sending marketing mails to customers. And we are planning to integrate 'Quickbooks' for accounts department within CRM. I am in search of these kind of tools or may be some apps which can be integrated within CRM so that it might reduce lot of manual work.

Kindly let me know if any one has used such kind of integrations in MSCRM and what are they and how it can be implemented.

Thanks,

sharmila