One of the more interesting config formats I've come across was an application that used an Excel file. Once you get over the horror of such a terrible decision, it was actually a quite interesting choice that allowed a fair few advantages. First of all each config subcategory was on a separate sheet making easy to navigate and find what you where looking for. You could use formulas to relate different config options (If you wanted A to be 20% of B you just set that in a formula). You could use drop-down for fields where there were only a limited number of valid values. You could include as much comments and documentation as you wanted (including diagrams and images) as long you only wrote in unused cells. And finally, my favourite, when configuring colours, instead of typing in RGB or hex values you simply changed the colour of the cell to the colour you wanted.

Now I would obviously never ever recommend doing this, but it was certainly an interesting and eyeopening experience.