One of the best things about windows is that you can see through them - and CrystalClear Interface thinks the same should apply in OS X.
CrystalClear Interface allows you to add transparency to just about any window in OS X, change the colors of window frames, and even select an entirely new set of default window styles OS X to use.
In reality CrystalClear Interface is not an application but an input manager for Cocoa applications. The amount of customization possible with CrystalClear Interface is a little overwhelming, but that's one of the few downfalls of the program.
You can choose from just about type of customization from making text contrast changes as you change your window's color, frame and transparency, automatically blur the background of windows, turn rectangular layout boxes into beautiful, color-matching rounded ones, and isolating active windows. The four preset themes - Aurora, In The Dark, In The Air and In The Garden - are elegant and to save yourself a lot of messing around, I recommend simply activating one of these.
If you're a big fan of Aqua and all the cool transparent effects it can create, you'll definitely enjoy using CrystalClear Interface.