I was going to recommend
TuDuMo. It was originally supposed to be a pc version of
TaskPaper, a plain text to-do app for the mac. It had many advantages: Plain text files. Very simple, and keyboard shortcuts. Not free, but very reasonable for the power.
But, now it uses XML files. And they're still decipherable, but a long way from the old format. In the old format, each todo was a line in the file. Tags were @tags at the end of the line. When a task was done, it was flagged with a - before the line. The app interpreted all of this into its interface, but it was beautiful. But now it's an example of what happens when someone decides to do too much with their original idea.
It's still a cool app, but I weep for what it was before.
Maybe I can make one that does what it used to do...