So I am a huge addon girl. I love them. And so I figured I should talk about a few that I'm not sure I want to ever play without. I'm trying to avoid the big ones (dbm, cartographer, recount, omen, those) in favor of ones that you might not have heard of.
1. Pawn. Since I am in a guild that wants loot decisions NOW NOW NOW, an addon that allows me to very quickly compare gear is great. It's also nice for leveling all those alts, since I tend to forget what stats I should like. That's what Pawn does. You rate your stats (or find a premade scale for your class or use the hunter dps spreadsheet to generate one) and it gives the gear a number value in the tooltip. Once you have it set up, it's great.
Pawn on Curse
The official Pawn forums, where you can find premade scales, among other useful things
2. ViperNotify. All this little addon does is ding (and put a message on your screen) when you're in Viper and full on mana. That's it. Obviously useful only for hunters, but it's a great reminder so that you don't go through 3 shot rotations on Patchwerk before you start to wonder why your dps is abysmal. And nope, I never did that. You can't prove anything. It was all Gariath's fault. Really. And if you want one that automatically changes aspects for you, Gariath recommends AspectChange. Never used it, but he likes it.
ViperNotify on Curse
3. Autotrack. This addon switches your tracking to whatever you currently have targeted so you get the bonus dps from Improved Tracking. Personally, I'm a little too used to always having Track Herbs on (ooh, an herb), so this addon is a nice one for me. Only problem is that it will switch you even if you kill something while farming/mining, so just remember that. Personally, I'm contemplating a Power Auras Classic reminder (if I can figure out how to make one).
Autotrack on Curse
4. Doublewide. This makes the quest window, gasp, doublewide. I cannot remember how I ever lived without it. Seriously. Blizz, please put this in as an option, PLEASE!
Doublewide on Wowinterface. Sadly, no curse for this one, but it's well worth it
5. Lightheaded. This addon pulls the comments from wowhead and displays them next to the quest in the quest window. Again, love this addon. Saves me having to go look. Probably not the best addon if you're worried about your lag, though.
Again, no curse. Wowinterface only
I run with way more addons then that, but I think these are nice ones that don't change things drastically. They're simple and require no setup except for Pawn.
If you're interested in UI addons, I plan to do those next. I'm thinking of calling it... UI for the raider, or um, does anyone remember what the default ui looks like? I probably don't, come to think of it. I'm so doomed next patch/expansion. Oh well.
(Gariath is a fellow guildie and RL friend.)