When you start to work on the game you have to remember about additional tools that may be needed, especially if you use a free graphics engines (for those who are curious – in most cases I use jPCT angine, and few times my own creation ;) ). One such tool that I knew that sooner or later I'll have to write was "level editor". It is important to consider this at the planning stage because you may find that the design of your game will never be finished because of the "small" oversights.
I most feared to start develop this editor. I felt that it will take me a lot of time to develop it, but fortunately it was not so bad - I chosed the simplest possible solution - application in the managed code with wrapper library for OpenGL 1.x (C# + Tao framework). Finally editor took me one day to develop :D and here you can see it in action: