I've abandoned TeuxDeux & NowDoThis, not because they weren't clever & functional, but simply because I wasn't using them well enough to be able to get the most from them.
I'm reverting to good old-fashioned pen & paper. Well, not quite, but to a Word doc at least, that I can print & carry with me, scribble on, throw away & reprint as needs arise. (I don't have an iPhone or a Blackberry, so maybe it's simply that backwardness that is at fault in this rejection of technological advancements.)
I have the page arranged in text windows whose lists ("Now do this", "Coming Up", "People to Meet", "Ideas" etc.) I can add to, strike through, delete from etc. I can even allow them to spill over the borders out of sight and as I delete items see the ones I'd forgotten about reappear.
I have a doctor friend who has always been an early adopter of technology (and a little more flush than me) who had a Psion PDA before such things became common. At the time I used to save a Word doc on my computer similar to that above, but one that was just one big long list of things I had to do etc., I called the file Psion as a way of mocking either my friend or myself, I'm not sure which. The latest version I have called OnePage after my good friend Michael FitzGerald's app OnePageCRM (which I am most definitely NOT abandoning or mocking, helpling me as it is finally sort out my sales pipeline.) (Maybe I should talk to him about turning my layout into an app!)
(I have another friend, running a great antique shop in the Powerscourt Townhouse Centre called Monte Cristo, who has all his contacts on one page including phone numbers & email addresses as required, printed out from his ancient Mac from time to time when the creases on the paper start to split apart.)