1) Curse client
2) Recount (Skada is a good choice also)
3) DBM (what I use) BigWigs and DXE (the one that was banned but is back I believe) are good too
4a) X-perl, been using it since van.
4b) Elkano's BuffBars
5) Dominos (it's like Bartender but different)
I'll also echo that Tidy Plates is possibly the best addon I've gotten.
GTFO is good for telling me to move.
Postal is great for mailing suff.
Bagnon for a bag addon.
Chatter for a chat addon.
Omen3 for threat.
ReforgeLite for doing all the math on reforges and giving a pretty good stat priority.
If you're looking for a guide for how to play ____ class the best place is IcyVeins