Very easy to do, there is a website:
Curse.com - WoW Addons WAR Addons Gamer Blogs Forums
which has almost 6000 WoW addons raging from UI overhauls, Economy addons for profiteering efficiency, boss encounter addons, everything you can imagine.
Installing the addons:
Download the addon from curse.
Extract it too your WoW folder 'Data C: World of warcraft: Interface: Addons'
Log in too your game, and activate them.
Blizzard encourages these addons.
