Agenda At Once
For example, when adding my first task, Agenda at Once asked me for a category--none of which I'd created. I just typed in a new category ("Reviews"), and, without any fuss or complexity, the program added it. This kind of obvious simplicity is sadly foreign to some software, so I am always impressed when a program just Gets It Right.
Functionality-wise, this PIM is aimed at the single user maintaining personal information. Although it has the capacity to assign tasks to individuals and track them, it is not an enterprise-level project management tool like Microsoft Project, nor does it aspire to be. (Neither does it bear a Project-like price tag of several hundred dollars.)
AgendaAtOnce allows the user to break a task into subtasks, nested arbitrarily deeply. Each task or subtask can be marked as partially or entirely completed. Colors can be assigned to tasks to help sort them, and there are many levels of priority which can also be assigned. Items can be given simple due dates, or assigned to a block of time (such as an appointment). For every type of data the user enters, there is an associated note field, allowing the entry of freeform information. (Text in note fields is searchable)
The program is fast, responsive, and has a low memory footprint. If you use scheduling/PIM software, it is worth checking it out for the 15-day trial to compare it to your current tool. If you don't, the program's intuitive ease of use might convince you to try your first--and perhaps only--PIM.