Edit: I no longer play World of Warcraft (actually haven’t played since June or July of ’05). I’ll leave this here though because my #1 search phrases are dealing with WoW. Once my finances get straighted up I’ll probably subscribe again, until then though you can only visit these somewhat out of date links.
I decided to put together a list of the various World of Warcraft UI Mods I use or have used so I can refer my guildmates to them without having them write down a ton of URLs.
Auctioneer (v1.10) is an addon that plugs into LootLink (included) to provide your tooltips with additional information on the average auction prices, number of times seen at auction and estimated vendor buy/sell price. This plugin is primarily for letting you choose what the best price to sell something for is.
Another mod on this site is great for enchanters because it will keep track of what items disenchant into.
There you have it, enjoy!