The general rule of thumb is that you can only move OS X windows by dragging their titlebar or a bottom bar like in Safari. Other applications like GarageBand allow for window edge dragging because Apple just felt like it.

Well, this Application Enhancer makes all apps equal. It’s called WindowDragon. Simply put, it allows a window’s entire structure to be used as move or resize zone. In other words, WindowDragon makes it possible to move or resize a window by clicking anywhere within that window.

Additionally, WindowDragon allows you to resize a window from any corner, and it allows you to drag all of an application’s windows in tandem.