Trello is simple, powerful, free and highly recommended. One of the secrets to their masterful design for both the user experience and the learning experience, is the fun and creative approach they take to progressive disclosure. The idea of progressive disclosure it that you release information or features gradually rather than all at once so as not to overwhelm the user or learner.
In order to keep the experience of finding your feet in the program fun and simple, Trello shows great respect (and support) for cognitive load by removing some of its features for the first-time user. As you gain proficiency rapidly by playing around with the features that are available, small contextualized hints begin to appear to suggest there's more to this program than you thought at first glance. Click to add a due date, for example, and the calendar reads "enable the calendar power-up to see your project in calendar mode".
Holding back on the calendar feature not only keeps it out of the way, thus allowing new users to focus on the basics first, but also, positioning it as a "power-up" bestows the extra value of an exciting bonus feature. The game reference is also a nod to the user's progress in mastering the program since in games, power-ups become available with mastery.
Kudos to Trello for the creative mix of progressive disclosure and game style for a great newbie learning experience.