I recently started using a small app for the mac called TaskPaper. Its basically a small todo list/outliner app, but with a big difference that sets it apart from the more complex competing products. Its file format is just plain text.... in fact, its so plain that it's probably very similar to what you would use if you maintained the same kind of list using a plain text editor. Let me show you a little sample from my own todo list at work.
Software update service: - Add public/private key sign. to connections @code - Add checksum after download and update @code - Add temporary file cleanup @code - Fix load issue with server side sockets @bug - Write readme file @doc
Thats really all there is to it. Having the file format this simple makes it possible to share your task lists using code repositories such as subversion with all the same benefits as regular source code files. In other words, its the perfect tool for maintaining todo lists in your code projects :-)
I could keep on blabbering about how nice and simple the interface is, or how cool the TextMate bundle for TaskPaper files is, but really... download the trial version and give it a spin. Remember, if you end up not buying the app when your trial is over, all of your work will be perfectly accessible in the plain text files TaskPaper stores its data in.