There are so many to list! But, I'll try to keep it simple.
Ualaa: Cartographer will never be updated by CkKnight (WoWAce)but he has offered anyone else to pick it up if they wish. You can pick up Mapster, which is what I'm currently using as my Cartographer replacement (thanks MiRai), but it is missing a few features that I loved in Cartographer: mouse over zones when you are in "continent" view to see their level, fishing levels, and dungeon's with levels in the mouse over. Other than that, and the fact that Cartographer had maps for ALL dungeons, not just Northerend (not sure on Outland dungeons, untried for me yet) and Mapster doesn't, everything else is great. You can still set your Foglight settings in Mapster to shade the unexplored parts, but still see through it.
QuestHelper: I use this addon exclusively for my questing needs, because for me, I do not think the Blizzard "lets try to take it over" even comes close. This is my personal opinion though.
HealBot: Got to have a healing frames addon, see my vid for exactly how I use it
![Wink ;-)](./images/smilies/icon_e_wink.gif)
XPerl: Somewhat like Pitbull, but I've been a long Perl-Classic then XPerl user, that I can't adapt to anything else. Shows either a percentage for health/mana or actual units, love that feature. Fully customizable.
Bartender4: I love having full customization at my finger tips, and Bartender delivers. I suggest this addon for anyone, whether you're using the default *cough*LAX*cough* action bar frames, or some other action bar addon, Bartender allows for full customization and profile saving so you can have even a multibox profile + a non-multibox profile if you're like me and sometimes run just 1 toon.
SexyMap: this is for minimap, since it appears that the author for simpleminimap is slowly grinding to a halt on that addon and I generally don't use alpha releases. Plus, SexyMap allows for moving the default objective tracker that Blizzard was kind enough to remove options for in 3.3.0 (Thanks Blizz!). Chinchilla Minimap I thought was a bit better, because of how I place my minimap, SexyMap doesn't let me place full against the side like I want, but it's worth it having that for moving the objective tracker. You can also make it all sorts of shapes to fit your overall GUI look.
Baggins: I use this with the All In One Sorted profile for all my toons. The only complaint I could even dream of having is that sometimes it's a pain for multiboxing because it's rare that all of your toons will have exactly the same items in all their bags. Your master toon will always have something the slaves don't, so it's hard to broadcast and click a quest item or something. I would try the one Lax has, but it just looks messy and no clue how to find something without having to search all the time.
![Wink ;-)](./images/smilies/icon_e_wink.gif)
Baggins allows for sorting like Consumables, Quest Items, Armor, Weapons, plus it allows me to use the Outfitter Plugin and create custom filters like my Druids' Boomkin and Resto gear's to keep them separate from the gear I loot in dungeons so I don't accidentally sell my gear.
![Very Happy :-D](./images/smilies/icon_e_biggrin.gif)
That's about all I can think of... and just thing, that was keeping it short for me!
![Rolling Eyes :roll:](./images/smilies/icon_rolleyes.gif)