Jade,
I think that for the todo-list application the new workflow-based and generalized Bug Tracker is a strong candidate. Lars has made the new Bug Tracker so customizable that it can very well function as a Ticket Tracker / Task Manager. In particular, versions and patches can be disabled, the pretty name of bugs can be set to tickets (or tasks or anything else), and tickets can be categorized along arbitrary dimensions.
The new Bug Tracker also uses the content repository so that adding search should be straight forward. This also means that the datamodel supports versioning of tickets.
