ToodleDo, the online Todo List. The basic service is free. You can pay for more capabilities in the enhanced versions, but that's between you and ToodleDo.
This app is very basic, and lacks any visual flair. It's large font text all the way. It will store your stuff for offline use and resync later.
You can define a ton of contexts, tags, classifications, types, and so on. As you can see in the pix to the right you can even define colors for entries in certain categories. You can easily implement GTD with this.
With a ton of other ToDo apps out there, this one ranks pretty low, having almost no "flair" to it. Still you are looking for a VERY plain ToodleDo client this may fit your bill.