Chardev.org is not only a fantastic web application, it also has a gorgeous user interface that is simple to use. You can either build a character from scratch or import your existing character from the Armory. Once you've made it that far, it's as simple as pointing-and-clicking to swap out gear, switch gems, redistribute talent points and add or remove buff effects. The display of thirty-five essential statistics remains fixed on the left side of your screen during most phases of editing process. At the present time, it only supports gear from level thirty and up but that's no real drawback. There are way too many customization options on this site to describe every one. You really need to experience it for yourself.
The Chardev site is a dream-come-true for people who enjoy playing with numbers, but hate doing the math. Check it out!