There are a few mods that do this. The two that I have used are Titan and FuBar. I have enjoyed FuBar much more.
Both are modular, meaning that lots of authors have written plugins for them, and you only have to use the plugins that you want. Titan comes packaged with the basics (like bag, money, clock, etc), while FuBar has them all as separate downloads. You may also see FuBar "packages" that people have put together as one download, too.
Both are available from this site. I personally recommend FuBar over Titan, but check them both out to see which one you like, or ask your friends.
As for how to download/install addons, read this FAQ if you haven't already:
http://www.wowinterface.com/forums/f...m#faq_download