Scrumpy - A simple, beautiful and agile project management software
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.
Planning Poker for GitHub - Story point estimation in GitHub with ZenHub
Nifty - Manage projects, work, and communications in one place.
Project Management Core Concepts - Book on core concepts of project management in plain English
duetask for Jira - Make task estimation transparent to increase project success