addons
(normally, i would say dont ever install a addon that has a executable file, however i am aware of the irony my first link is a bundle of addons with installer)
cosmos : cosmos is a collection of great addons with a simple installer & updater
it contains loads of seperate addons but i like it because it bundles together with a map addon to use later, and khaos. which is great so you can create addon-profiles for each character to load & like mentioned .. loads more
(details) .. and it will show coordinates on the map
lightheaded
ingame quest info with selected comments from the wowhead website
with the combination of cosmos and lightheaded you should be able to click on a coordinate in one of the comments and get a arrow on screen
you can still show a dot on the map for a specific coordinate with the command
/mntloc x,y
(where x and y are numbers ...ofc)
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other great addons.. or at least . i think they are
auctioneer a great tool when you ever use the auction house (dont we all ?)
crowbar when you pickup boxes/clams/bags/etc that you can open you get a nifty icon so you dont have to search your bags for it, then open it from there
autoprofit
one button to sell all the gray items in your bag on the vendor window :)
loooads more .. but ill keep it like this for now
the raiding people will already know about damage and threat meters ;)
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couple of big addon sites
http://files.wowace.com/index.html
http://www.curse.com/
both released a way that you can run a program and it will scan your addons to make sure you have the latest known version :)
there are several other big ones