TaskUnifier is a free task management software based on GTD (Getting things done) methodology.
Main features:
supports subtasks, tags, folders, contexts, goals and locations.
supports plugins
synchronize your data (tasks, notes, folders…) with third-party websites
customize the appearance
requires Java 1.6 minimum.
works behind a proxy
supports notes
sort and filter tasks
Download TaskUnifier
For implementing GTD you can use this this application:
http://www.Gtdagenda.com
You can use it to manage your goals, projects and tasks, set next actions and contexts, use checklists, and a calendar.
Syncs with Evernote and Google Calendar, and also comes with mobile version, and Android and iPhone apps.