I have been using a minimalist to do program for a while. It's called TodoTextMac. A fuller description is found here. If you would like an explanation of the principles behind TodoTxt, I recommend this Web site or the Web site by the originator of the format.
TodoTxtMac is of course for the Mac. You can also get an application for the iPad, called Todo.txt Touch, for Windows (Todour), as well as for many platforms. I have only used the mac and the Windows version. They work well, and their files are fully compatible with each other.
The basis idea goes back to Gina Trapani, and it started out as a very sparse command line application. I like simple, but find the original version a bit too sparse. TodoTxtMac and Todour hit the sweet spot for me.
Oh ... and did I mention that the applications are free?