After nearly a year of work, WoWInterface's UI Manager is finally in beta testing. It's cross-platform and written in a combination of languages to ensure OS compatibility. It also includes a Vista manifest to ensure Windows Vista users can use it without problems caused by User Account Control (UAC).
Two pages exist for it, click on the relevant one to learn more!
MSI Setup Package (Windows Users)
Generic (Linux/Macintosh Users)
Please post bug reports either at this portal or on the comments page.