OmniFocus is a popular Mac productivity tool inspired by GTD. It offers an iPhone version that syncs well with the desktop application.

Unfortunately, OmniFocus has one glaring flaw in our estimation: it only supports a single context per task, so it is only suitable for simple GTD, and it is unsuitable for TRO (Total, Relaxed Organization, compared to GTD here). See this discussion thread on the OmniFocus forum for some insights about why multiple contexts are important:

We hope this issue will be addressed in the future.

See also this important discussion (in this forum) about OmniFocus:
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