I use many, many addons. I like playing around with them and trying out new things. There aren’t many addons, however, that I would class as ‘essential’ for my UI.
One which is, is Bartender. Actually – a bar mod is essential, it just so happens that Bartender was the first I tried and I haven’t had any problems with it.
The reason I find it essential is because I find it difficult to set up my bars for druid forms, plus lots of key modifiers, with the standard UI. I want a bar that is the same for all forms, which I access with <ctrl>. My <alt> bar is similar on all forms, but slightly different. My <nomod> bar is obviously different for different forms.
This tracks onto how I arrange my bars for my alts. I tend to use buttons 1-4 for ‘standard rotation’ abilities, 5-7 for less used longer cooldowns or panic buttons, and the <alt> bar for CC and abilities I tend to only use for pulling. My pally also has more panic buttons there because OMG they have so many.
I like being able to play my alts without trying to remember what their key damage abilities are. However, I don’t tend to play DPS characters much so I’m not sure I’d be able to fit all of the warlock abilities, for example, into the same system.
Another essential addon (well, set of addons), is a click-casting mod plus raid frames. I’m currently using VuhDu and Clique (Clique is slightly redundant in this setup, but I sometimes click-cast on unit frames), but I tend to swing between VuhDu and Grid. I didn’t like Healbot much when I tried it – I think it wasn’t configurable enough for me.