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Another option is MyLifeOrganized. It's what OF replaced for me when I moved from Windows to OS X.

MLO runs on Windows, iOS, Android, Blackberry, Pocket PC. There is no OSX version, which is a bummer. But it runs well in a VM and also in an emulator like Wine or Crossover.

I went back to using MLO when I decided to go with an Android Smartphone rather than an iPhone. I looked high and low for a way to get OF and Android to work together in a satisfactory way. The scripts (already mentioned in prior replies) to sync to Todo.txt was about the best option I found for sticking with OF and syncing with Android.