Asana is web based Project Management Tool. It is well designed for managing team workflows and task-management. Fast and responsive. Free version supports up to 15 team members. Lovely HTML5 design which is feature-rich. Also there is a mobile version of the same.
We are using Asana from last 4 yrs and are quite happy with it (Team had tried a lot of other tools before sticking to Asana). Free version has few limitation but nothing which a small team cannot survive without. All basic core functionality such as due dates and the ability to upload attachments, as well as unlimited tasks, projects, and conversations are available in free version.
One of the most used feature for us is assign task directly from email. It is the most easy way to assign and complete task. You can check out and register for a free account at www.asana.com.