Yes, such nice tool really exists and it's name is Remember the Milk.
At first sight, RTM is not extremely visually appealing, but very nice and very usable Ajax application. After some time using it, you may better appreciate the "less is more" design. The offline access is thanks to Google Gears magic. Just flip the small green arrow (which will download your data to local cache) and voila - you can go offline, keep editing or adding tasks - your changes will be remembered. Later on, after going online the changes will be synced back.
The RTM has another killer feature - it integrates very well with Google calendar. Your tasks with due dates (which all meaningful task should have) will appear in you calendar without needing to go to RTM site. You can even edit them from there. For details, see the help.
RTM has pretty much everything what most people may need: projects, lists, tagging, locations (with map integration), contacts - with task delegation, and of course, API. Give it a look ...