OmniFocus is the best task-organizing device I have ever used. For idea-organizing, see Napkin Notes. But back to OmniFocus… the best part is the completely intuitive iPhone app, although there's a version that works on your Mac, too. Now, there's also an iPad app as well. For years, I synced via my MobileMe account, but Apple is closing down MobileMe June 30 (2012) so I've moved to Omni Sync, a free service by the authors of OmniFocus.
Here's why the iPhone app is so important: every other organizing device I've ever used is limited by the fact that it's not with you all the time. But my phone goes everywhere with me except the shower, so if something occurs to me, I can note it.
The OmniFocus people make the huge point that there's only so much you can remember... much like RAM on a computer, your memory simply gets full. This "RAM jam" is one of the biggest impediments to productivity that can occur, they assert, and I agree... When you're always tormented with "to-do's" that keep dancing in and out of corners of your mind, you're far less productive.
Their solution is to do a "mind sweep" over a period of 2-3 days of all the things in your life that you have any degree of commitment to someday getting done. OmniFocus makes it really easy to then put your whole life in order... which is sure to generate a surprise or two.
The really cool thing is that you can organize your life into either projects or contexts. The problem with projects is that you almost never finish one, so they hang over you. But OmniFocus breaks them down into individual tasks... Bite-size chunks, if you will.
The other cool thing is Contexts. A Context is something you need to do the task. Maybe it's your Mac. Or your office. Or a person. Or a store. So you can find all the contexts available to you right now, so that you can see what you can be doing this moment. You can even hit the "Locate" button and your iPhone will find where you are and show you all the contexts in your vacinity! How cool is that?