I've been using Next Action by TrimPath a lot lately. It's a to-do list manager based on Getting Things Done. It lets you mannage your actions, in an off-line fashion. It's a very nice adaptation, and is my current favorite program for this sort of thing.
Both the strongest and weakest point of the program, are that it's stored offline, in an HTML file. Every time you make a change, if you navigate away from the webpage, you'll lose the changes, unless you save the document. If you run Firefox, you can open in a new tab, so you don't use the forward/back buttons to inadvertantly navigate away. I'm usually online, so the ability to use it offline hasn't come in handy yet, but I can see where it would be, at an airport or traveling.
It would be nice to be able to use it anywhere there's Internet access, if it was hosted somewhere, but at the same time, I think you'd lose some of the utility if you were waiting for more than a millisceond or two to save something.
I'd highly recommend it as an application to enhance your productivity.