Onesheet CRM - The Simple CRM for Google Sheets
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.
Zendesk Sunshine - Open, flexible CRM platform built on AWS
Freshsales CRM - A full-fledged CRM for high-velocity sales teams.
Gmail CoPilot by cloudHQ - Easy CRM: View all past conversations with an email contact
Pipedrive - Sales pipeline software that gets you organized. Helps you focus on the right deals, so easy to use that salespeople just love it. Great for small teams.