GTD application for both Mac and Windows

It sounds like what you want is a web app. Remember the milk does everything you want and has offline syncing.

"Web apps are the best way to get cross-platform sync. (It isn't really sync, you just access the same db from different OS).

I use Toodledo, which has iphone app(s) and also lets you add sidebars and gadgets to your gmail, google calendar and igoogle pages. The iphone app even works when you have no internet access.

If you don't like the idea of a web application, I would suggest you take a look at chandler which has cross platform clients that can sync over the internet, or some of the tiddlywiki based GTD-mods like GTDTiddlyWiki plus.

Tiddlywiki is basically an application written in html and javascript, so it consists of a single html file that you carry with you, all you need to use it is a modern browser."