Nozbe is, succinctly, a Getting Things Done (GTD) implementation

People interested in Nozbe may want to also investigate Conqu, a similar app but with different design trade-offs. Various things that I find annoying or broken in Nozbe are not that way in Conqu, and vice versa, and the two have slightly different feature sets and UIs. (Notably: Conqu's app works almost exactly the same on all platforms (given a large enough screen), vs. Nozbe's many slightly different apps.)

Conqu is also cheaper on an individual basis, and can be used for free in an unlimited fashion on any single device: you pay only if you want to be able to sync between devices and email yourself tasks.