InGrav Tasks

InGrav Tasks offers an efficient way to work with tasks on your mobile devices
is a simple to use to-do list app that can help you plan, track, and get more tasks done. You can create unlimited tasks, set status, give due dates, assign tasks to team members, create to-do list items in tasks, and assign them separately to team members. Each assignment is accompanied by start and end dates, statuses, priorities, duration and estimation time

it's very well-designed and user-friendly It's designed for the iPhone and can be used on iPad

Key features:
Set start and end-date to define task deadlines
Send and receive team messages
Task assignment to team members
Workspaces to organize everything.
Team collaboration: comments, attachments, mentions.