I use Sublime Text 2 as my primary text editor, and I really like PlainTasks plugin (https://github.com/aziz/PlainTasks) as a way of keep tracking the things I need to do, things I would like to do, and, most important, things I've already done (just to have some sense of accomplishment).

In a side project with so few hours per week, I think anything more complex than this is overkill.