With Trello you can:
• Create boards to organize anything you're working on.
• Use them solo or invite coworkers, friends and family to collaborate.
• Customize workflows for different projects
• Add checklists of "To-Dos" on cards
• Assign tasks to yourself and coworkers
• Comment on items with your friends
• Attach files from Google Drive and Dropbox
• Upload photos and videos
• Reply to comments from Android Wear
• Display cards in a calendar view with the Calendar Power-Up
• Work offline and boards will sync automatically when reconnected
Trello is free to use forever.
"Trello is one of those few apps that actually balances both a simple, elegant user interface and powerful, flexible features." - Lifehacker