This question has been answered by Bruno17. See the first response.
you could use formit with a formit2db - hook-snippet, to store posted todos from a frontend - form to a custom-table.
you can show the todo-list and the todo-details with something like rowboat or migxLoopCollection(part of the MIGX-package)
If you need backend-management for this todo-list, you could create a CMP (Custom-Manager-Page) with help of MIGXdb.
Tutorial to get started:
http://rtfm.modx.com/extras/revo/migxdb/migxdb.tutorials/migxdb.create-doodles-manager-with-help-of-migxdb