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.
Camunda - The Universal Process Orchestrator
Process Street - Create beautiful rich process documents in a simple to follow checklist format. Fast, free and incredibly simple to use.
ProcessMaker - ProcessMaker is an easy to use BPM and workflow software solution. It is used to design, automate, and deploy business processes of any kind.
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.
jBPM - jBPM is a flexible Business Process Management (BPM) Suite.