[JeffJenkins]

If you're on a Mac, you could try OmniFocus. It's a pretty decent implementation of Getting Things Done (the core of which is what you're describing). I've been using it since it was in beta and I'm really happy with it.

It lets you make folders, projects, and actions. Actions can have start/end dates, repetitions, and contexts. The idea of contexts is that you should be able to see what actions are available to you at any time in any place. Actions can have sub-actions and you can specify whether the sub-actions have to be done in sequence or can be done concurrently. You can also set up a global quick-entry hotkey which will let you enter an action and have it placed in your Inbox so you can organize it later.

There's also an iphone app, but I haven't found it teribly useful. OmniFocus has the ability to grab tasks from Mail.app in a special (user-defined) format and I found that works as well for adding new tasks as anything.

The downside is that it's $80. I got it for $40 when it was in beta, and I think it's a much harder sell at $80. If you aren't interested in this, or aren't on a mac, just google "getting things done" or "GTD"