An editor that might no be too well known in the web world, Unity3D's editor, has an awesome way to edit code on the fly. Public variables are represented as generalized form controls like sliders and dropdowns. Very nice for the later phases of development. Tweaking the timing and speed of an animation or the effects of some physics float can be a pain without being able to change on the fly. I wish the web tools had a similar setup.

http://docs.unity3d.com/Documentation/ScriptReference/Editor...