While OmniFocus is clearly on one side of the spectrum in terms of functionality and possibilities, TaskPaper is on the other.
It really is one of the go-to todo list application for people that need less and enjoy plain text files. When playing with it, and I keep doing this every now and then, it always intrigues me with it's simplicity. I just manage to many tasks to make it work for me. However, there are a number of people that use OmniFocus side-by-side with TaskPaper, e.g. to manage detailed project tasks (for example for a coding project) with the latter.
With excitement I came across this post which leverages TaskPaper's extensive AppleScript interface to make it interact with Postbox, the only true email client alternative for Mac OS X left.
Jesse Grosjeanesse, the developer behind TaskPaper, is currently working on an already popular evolution of plain text and markdown editing called FoldingText (in stable beta) which will also, I am sure, change how TaskPaper will work in the future.