For Addons, check out the various addon sites, I use...
http://www.curse.com/
http://www.wowinterface.com/
for Macros, start here...
http://www.wowwiki.com/UI_Beginners_Guide#Macros
and the official Blizzard forum for discussion about addons and macros is here...
http://forums.wow-europe.com/board.html?forumId=11098&sid=1
If you're asking for specific examples of addons I use, here are some examples...
Armory - caches char details so I can view char A from char B on the same account
Atlas - has detailed dungeon and BG maps with boss locations and loot tables
Auctioneer - helps long term monitoring of auction prices
BankItems - another char cache addon which also caches guild banks
BGDefender - makes it easier to announce how many enemy at your current location
BuffWatch - monitors buffs in your party/raid and shows when they've expired
Carbonite - a superb quest tracker and so much more. Best. Addon. Ever.
DeadlyBossMods - monitors boss fights and announces events you need to be aware of
Decursive - frame based party/raid decurse addon
Elephant - logs all chat channels you use for future reference
FuBar - very nice panel interface which supports plugins
GemHelper - helps determine if there is a gem of colour X that does Y
HealBot - complete UnitFrame replacement for dedicated healing
Macaroon - macro-based action bar addon
Omen - threat management
Outfitter - easy management of equipped items
Pitbull - UnitFrame replacement
PowerAuras - monitor you and party/raid for buffs/debuffs
Recount - hugely detailed combat monitor, collecting damage data etc
ScrollingCombatText - does what it says on the packet but much better than Blizzards ripoff
StatusBars - puts my life/mana/rage bars and target life bars near my char
WIM - Instant Messenger style whisper addon
YATA - Shaman Totem management addon