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