Fan of RememberTheMilk GTD and trying to find a way to use in Mac? You are not alone. Devs haven't made any announcement of desktop app. However there is an Outlook plugin for pro users. Had to say the webapp is nicely done, simple and still packs lots of features. Huge plus is keyboard shortcuts. Good thing is RTM supports iCal to which other calendar applications can subscribe to. It does not work both ways, can create new tasks.
There are two ways to half-integrate RTM in Mac,
1. Subscribe through Mac's in-built iCal. Simplest approach but iCal had to kept open to refresh changes in RTM which I don't prefer.
2. Second approach involves Google calendar and as a bonus will show RTM tasks in Android calendar application. RTM provides GCal widget but go ahead and subscribe to RTM iCal. Now RTM tasks will show up in application that support Google Calendar, one such app is QuickCal for Mac. QuickCal is a menu bar application and in one keyboard shortcut all RTM tasks will be listed.
One thing missing in above two approach is adding tasks to RTM. Well, there is a Ruby library that supports RTM api and can be used through Alfred to add tasks to RTM. Instructions here.
For full task management I still use RTM webapp but above two approach works well 75% of time without having to switch application.
Tips for Indian Users:
RTM supports reminders via SMS. Unfortunately only Reliance India is supported and RTM has no plans to expand it to Airtel/Vodafaone/Aircel which most use. However Twitter can send SMS for any direct message across mobile networks and RTM supports reminders via Twitter. Make use of it.