The first addon I ever installed:
DagAssist. - I use this for my professions and hearth / ports / mass ress etc. There is another addon which can make it a broker addon and I just find it so useful.
Geist - For things I don't always want showing on my action bars, don't want to keybind, but still need access too easily. I use it for abilities that aren't a part of my normal rotation, but that I might need in a hurry.
QBar For all of the quest item things. This is especially usefull if you've got an addon to move the watch frame and it being the delicate flower that it is it won't let you click items in the watchframe any longer.