I have been using Google Tasks to make to-do lists and am finding it really useful.
The interface is quite basic. You can add tasks and indent them to give it a bit of basic structure. It gives you multiple lists which at first seemed a bit cumbersome; I’d like to see an overview of all tasks. Having used it for a while I think it’s actually not such a bad idea.
Tasks is integrated into gMail and iGoogle, but I mostly use it via the Firefox sidebar (use https://mail.google.com/tasks/ig) and gTasks on Android.
It’s not as advanced as some services, but it is fast and simple.