OmniFocus is a high-powered, flexible, feature-filled task-management app that's based on the widely used Getting Things Done (GTD) productivity method, but it's designed to accommodate a variety of work styles. OmniFocus uses a GTD-esque approach ("Capture, Organize, and Do") to handle actions, projects, and contexts. You work in a relatively clean, two-pane interface, with folders and projects in the sidebar and grouped actions in the main outline (where you add, edit, and check off actions).
Within this interface, it's easy to set up versatile contexts for your actions, distinguish between sequential and parallel actions within a project, and drag and drop actions, projects, and contexts. The sidebar also contains the In-box, a "holding area" for incoming actions, and you can enter new actions here, or from anywhere using a system-wide shortcut. OmniFocus even helps you add actions in several other ways, including via e-mail. There are also lots of different ways to view your workspace, and an improved Perspectives menu lets you save the ones that you use the most (including a set of editable defaults for common Perspectives, like In-box and Flagged).
Overall, OmniFocus's many features and highly tweakable interface deliver a lot of horsepower, making it one of the best (and best documented) GTD applications available. All that functionality comes at a price, though--and there many cheaper options out there--but GTD devotees should definitely give OmniFocus a try.