Toodledo is a great online application with a stupid name.
I use Toodledo for my GTD ToDo-lists, and it works great. There is a nice app for the iPhone, but they also have a great mobile site that you can use from any phone (www.toodledo.com/slim).
It supports all the things you need for GTD: contexts, goals, etc.
Also, you can set up Toodledo for emails. Send an email to your secure Toodledo email address, and the email is automatically converted into a task.
I often use the Toodledo email to create tasks in my ToDo list directly from my mailbox. When I need to remember to do some action on an email I just send the email to Toodledo. The task is set up within seconds, and the email is stored with the task. When I go to the task in Toodledo all the information is right there, in the task.
If you are struggling with your to-do list, give Toodledo a go.

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