[gazhayes]

If you just want a web app, there is a *very* good one that leaves Omnifocus in the dust in terms of GTD implementation (OF only takes care of runway and 10,000ft). Nothing web based is going to be as sexy or easy to use as omnifocus though - I've tried everything and there is nothing that beats the usability of omnifocus.

The web based implementation of GTD I am referring to is called GTD-PHP (pls google it) and is open source. You need some of the extra modules to get things like email=>inbox etc.