Planning Poker - Open source implementation of planning poker game (collaborative online estimation/voting tool).
Nifty - Manage projects, work, and communications in one place.
Firepoker - Agile Planning Poker powered by Firebase and AngularJS
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.
Taiga - Project management platform for agile developers & designers
UpDiagram - Free agile project management tool for scrum team