Now that there is an API for Google Tasks (tasks in Calendar), do you know a tasks app with the following features?
I think Astrid is open source but you might want to check better because I'm not sure.
I don't know if it's open source but I use GTasks and I am very happy with it. It syncs with Google Tasks, has a nice UI, and you can add a new task by just giving it a name.
-= EDIT =-
I've got it! I think this works... Shuffle for Android