While Salesforce and Zoho are popular, there's much better solutions out there for Google Apps CRM.
Instead of using multiple apps to manage CRM, projects, billing, and other needs it could all be centrally consolidated and managed in a single google apps integrated system, like WORKetc. WORKetc puts all of these features into one business management app capable of managing an entire business - and it integrates with google apps. The functionality you can achieve right from Gmail is particularly powerful.
The difference between regular CRM apps and WORKetc is, with WORKetc you're managing your client's entire lifecycle through one system, from lead to project collaborator to billing. The client histories list essentially everything you've ever done with a contact, including all correspondence, related activities etc is one filterable page.
You can read about it here: http://www.worketc.com/features/google_apps_integration
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