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.
Leantime - Lean project management for startups & small teams
Asana - Asana project management is an effort to re-imagine how we work together, through modern productivity software. Fast and versatile, Asana helps individuals and groups get more done.
Taiga - Project management platform for agile developers & designers
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
PlanITpoker - PlanITpoker is a cool on-line planning poker app that helps Agile project teams estimate projects easily.