Well there is windowblinds from the folks that made iconpackager, aston shell and my current favourite stylexp. Aston is the most drastic change but makes your GUI more efficeint not less efficeint as alot of skining tools do(It also has some very cool themes once you get used to the different layout)
Windowblinds, i dont really like it, has some cool themes but isnt very efficeint and munches system resources.
Stylexp its somewhere between aston and windowblinds, it doesnt use anymore resources than the windows themes as it essentially replaces them with its own cooler ones, also has other cool options like boot and logon screens, new screensavers and new wallpaper, as well as icons and mouse cursors.