Two that I would recommend:
Toodledo www.toodledo.com
Can be used for free or for a $15/year subscription you get additional features. Not specifically designed for GTD but is such a flexible system that you can set it up to handle GTD methodology. It syncs with a number of iPhone apps (including Toodledo's own iPhone app) and one or two Android apps. There is a third-party Outlook sync tool, and you can do neat things like send emails with attachments to your inbox, add tasks via twitter, get reminders via SMS, etc.
Bookmarks