ActionGear has been out for a while, but I think it deserves a mention as a potentially great task-management app, falling somewhere between iCal or a bare bones outliner such as TaskPaper, and the robust end of the scale where Things and OmniFocus are duking it out.
Within ActionGear's simple interface, you'll find a sidebar with your Inbox, (nestable) Groups, and Smart Groups. The main focus of the interface is a task list in which tasks can be nested within their groups (folders). Each task can have flags, notes and attachments, and there's a tagging system which allows Things-like flexibility. You can use it as a basic to-do list, or make the Groups into projects and tags into contexts and go all GTD with it. The Smart Groups can emulate, to some extent, the Perspectives you'd find in OmniFocus.
Overall it seems like a good solution for those who need a little more structure and power in their task management than what can be done with iCal, but don't really need all of the extra features of OmniFocus. If that's you, give ActionGear a shot with the free trial. If you dig it, it's $29USD for a license.