I've tried many list managers for GTD and my current favorite is Remember the Milk. RTM has offline use with Google Gears, integration with Gmail and Google Calendar, the ability to add tasks from a phone call using Jott, shared lists and mobile access.
What makes a good GTD list manager?
Having a total life reminder system is a key to GTD and a trusted list manager to track projects and actions is one of the first choices for people to make when implementing the system.
Unfortunately, some of the programs out there that are trying to be "GTD list managers" miss the mark not by what they didn't include, but by what they did include. Some of them build in too many convoluted features, that in GTD terms make the whole thing more complex than it needs to be.