To begin, you addon newbs may be wondering how to go about finding good, reputable addons. Our own Addon Spotlight feature here at WoW Insider is a great place to start your search. We've reviewed many popular addons, and our readers have chimed in with lots of great advice and feedback about addons. You can also ask fellow gamers, or better yet, your guild. Some guilds require certain addons for members, but regardless of your own guild's policies, your guild-mates may have some good addon advice. There are a few primary sites you can visit to find addons, each of them doing essentially the same thing in different formats.
Another way to find addons is to search for them by name using a search engine like Google. Many developers keep the most current version of their addons on a native site. Examples are Auctioneer, CTMod, Cosmos UI and Healbot Continued. For you Ace advocates, I should mention the Ace addon system, which many players choose because Ace addons play well with each other and used shared libraries to minimize memory use.