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.
Sortd - Rated the #1 App for Gmail
HubSpot CRM - Free CRM software for small businesses.
KanbanMail - A Kanban board for your emails.
Streak - Manage customers inside Gmail
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.