Crmble - Easy peasy crm for Trello
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.
Onesheet CRM - The Simple CRM for Google Sheets
GoodGantt - The best Gantt chart for Trello®
SquareOne - Email. Stress-free.
Paws for Trello - The best way to use Trello on your desktop