TaskPaper saves its documents as plain text. They can be opened, modified, and created by any application that can work with text files. In fact, it is fair to say that TaskPaper is both an application and a lightweight text-markup system specifically for to-do lists. It is pretty easy to support the TaskPaper file format, and it has been done for several text editors.
Some months back, I created a ToDo mode for SubEthaEdit that supports the TaskPaper format. Today, I finally got around to making it available for download. The mode supports syntax highlighting and has scripts to automate creating new tasks and projects, marking tasks as done, and archiving completed tasks. Tags are detected and highlighted, but there is unfortunately no way to do the outline-processor-style hoisting of particular tags. To aid in managing multiple tasks, project names appear in the function popup menu.
It is also possible to modify how the mode handles marking tasks as completed and archiving them. This requires an additional script to open a plist of environment settings; install this script in the scripts folder for SubEthaEdit (if you're not sure where that is, use Open Scripts Folder under the scripts menu in SEE). Two relevant keys can be set,
SEE_TODO_ARCHIVE_DONE. The values should be set to shell commands that implement the desired behavior for marking tasks as completed and for moving completed tasks to the Archive pseudo-project.
One possibility is to pass different command-line options to the scripts that implement the default behavior for the mode. For example, you could set the value of
'"$SEE_MODE_RESOURCES"/bin/markdone.sh -c -t'and the value of
'"$SEE_MODE_RESOURCES/bin/archivecompleted.awk" -v Mode=c'(including the quotes in the values). With these flags, tasks are no longer marked complete with a
@donetag, but instead the leading hyphen is turned into a plus sign, giving a sort of check-off effect instead of a tagging effect. Be aware that this breaks compatibility with the TaskPaper application.
Update: The ToDo mode is available on the Coding Monkeys website.