Mac OS X already does some of this out of the box, in case you weren't aware: hold down the Command key (Apple key) and you can move a background window by its title bar (or, if it's a metal window, anywhere by its frame) and resize it by its resizing corner.

MondoMouse may make this a bit easier, because the cursor can be anywhere within a window's bounds. I haven't used it for an extended period myself, though, so I can't really comment on it.