Agenda - A date-focused note taking app for both planning and documenting your projects.
Sortd - Rated the #1 App for Gmail
ToDoBook - Turn your Facebook (& other social media) News feed into a to-do-list.
KanbanMail - A Kanban board for your emails.
Todoist 10 for iOS - A completely new Todoist for iOS
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.