NetHunt CRM - Turn Gmail into a smart CRM
Followup.cc - FollowUp for email is a lightweight productivity suite that keeps your conversations and tasks from slipping through the cracks. Set perfectly timed reminders that arrive at the top of your inbox, right when you need them.
Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.
HubSpot CRM - Free CRM software for small businesses.
Inbox by Reply - Handle your hot sales emails faster
Streak - Manage customers inside Gmail