Besides opening up applications and peeking around, AppHack can also perform some theming duties... for example, it can replace icon sets. And here's a developer who clearly doesn't mind having his own medicine eaten: AppHack is distributed not only as freeware, but as open-source freeware, so you can take a look at its code to see how it does what it does. Great stuff... highly recommended for your inner (or even outer) geek!
Version as tested: 1.1.