Many launchers support different themes, but you have to download them from the Market as if they were apps, which can be incredibly cumbersome. N3xGen organises them into their own little "theme store", making it easy to pick, download and apply a theme in no time.
N3xGen doesn't add any new themes, it just takes ones that are available in the Market and organises them -- you can search by developer, free, paid and even categories like "Blue", "Dark", "Gingerbread" or "Icon Pack". Right now, it supports ADW Launcher, LauncherPro, Go Launcher, CyanogenMod, and the T-Mobile Theme Changer.
N3xGen Theme Manager is a free download for Android devices.