If you're a big fan of Remember the Milk for basic to-do lists and location-aware reminders, you'll be pleased to see how easily you can tweak it to contain your entire GTD workflow.
Over on the official Remember the Milk (RTM) blog they have a guest post by an avid RTM user on setting up a GTD system with RTM. The system takes advantage of RTMs tagging and location system to build daily task lists, separate by context, and even house the someday/maybe.
Where the system really shines, however, is the use of RTM's "Smart Lists". Thanks to the advanced search operators available during Smart List creation you can easily make lists that cut through greater contexts like a custom errand list that only shows errands you can run around your office during your lunch break or a call list that only shows personal calls you would make from your personal mobile phone while hanging out in a waiting room.
Check out the full and quite detailed post at the link below. If you're currently using Remember the Milk as your primary GTD system we want to hear about it in the comments.
Advanced GTD with Remember the Milk [Remember the Milk]