I keep my To Do items in a simple text file, for lots of reasons (including the Prags advice to "Keep knowledge in text" from the Pragmatic Programmer). Because I'm a developer, I have lots of tricks to manipulate text, especially in TextMate. Thus, my to-do stuff consists of loosely categorized lists.
The other day, I found a simple but very cool TextMate bundle that provides a couple of commands and syntactic eye candy (the visual equivalent of syntactic sugar) for simple to-do lists. Now I can keep my to-do list in plain text and still get the pretty categorization and other stuff I want. More support for my contention that TextMate is the Swiss-army chain saw of Mac OS X.