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