I read once that it's the programmer's holy grail to find the ultimate todo list program. My recent holy grail has been to distribute work efficiently across a number of machines, and todo list maintenance is an awfully serious chore in that world.
So while I don't think this is the ultimate, here's a cute hack:
1. Get an IMAP server.
2. Write a draft (with empty "To:" so you don't accidentally send it)
3. Save the Draft to the server.
Now you have a distributed document, with rather bad locking.
So it's not great, but it's a basic distributed notepad, and it took you five minutes to make. :)