Sortd - Rated the #1 App for Gmail
NetHunt CRM - Turn Gmail into a smart CRM
KanbanMail - A Kanban board for your emails.
Immediately - Mobile sales email and CRM management for Gmail & Salesforce
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.
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.