Addons!

Here is a list of addons that I find useful, plus a few others:

Quest Helper
Simple quest tracker.
http://wow.curse.com/downloads/wow-addons/details/quest-helper.aspx

Cartographer
For easy manipulation of the world map, includes coordinates, unexplored areas, instances, etc.
http://wow.curse.com/downloads/wow-addons/details/cartographer.aspx

Tradeskill Info
Provides all kinds of info about items that can be used in tradeskliing (on mouseover).
http://wow.curse.com/downloads/wow-addons/details/tradeskill-info.aspx

Auctioneer
Auctioneer includes some nifty info for enchanters: Enchantrix. It tells you what items will disenchant into.
http://wow.curse.com/downloads/wow-addons/details/auctioneer.aspx

Ackis Recipe List
Enables a "Scan" button in your tradeskill window and you can track what recipes you have and what you are missing.
It even tells you where you can acquire recipes, and how.
http://wow.curse.com/downloads/wow-addons/details/arl.aspx

Recount
Damage and healing meter.
http://wow.curse.com/downloads/wow-addons/details/recount.aspx

Rating Buster
Item comparison tool using percentages. Makes it easier to decide which item is better by including percentages.
http://wow.curse.com/downloads/wow-addons/details/rating-buster.aspx

Omen Threat Meter
Provides values of relative threat level. Usually only critical in raiding situations.
http://wow.curse.com/downloads/wow-addons/details/omen-threat-meter.aspx

Deadly Boss Mods
Displays timers and warnings of raid bosses' spells and abilities.
http://wow.curse.com/downloads/wow-addons/details/deadly-boss-mods.aspx

Carbonite
A lot of people like this one, I think it's a little awkward, but I admit it can be very useful.
http://www.carboniteaddon.com/

Healbot
Healing management interface. I've heard that some prefer PerfectRaid.
http://wow.curse.com/downloads/wow-addons/details/heal-bot-continued.aspx