Raid frames
Grid
"Grid is a modular, lightweight, and
screen-estate saving grid of
party/raid unit frames."
A very good raid frame which is very
useful for healers. (Clbull)
Note: it can take a lot of fiddling to
set up how you like it, but lots of
tutorials exist for it and once you
get the hang of it, it's easy.
(lilserf)
Click-bindings
Clique "Clique is a simple GUI that lets you assign click-casting
for any number of unit frames. Click
casting allows you to define the
behavior the game takes when you click
on a frame. This can be casting
spells, running a macro, using an
item, or something as simple as
changing targets or assisting a unit."
Excellent when used with Grid to let
you bind various clicks (right, left,
middle, shift-left, etc) to different
spells. (lilserf)
Cleansing
Decursive "Decursive is a cleansing mod intended to render
affliction removal easy, effective and
fun for all the classes having this
ability."
I use Decursive, and I believe it (or
a similar addon) is not only helpful,
but imperative. (Hhoky)
All-in-one
VuhDo "VuhDo is a raid monitor similar to CTRaidAssist or
Blizzards built-in raid frames.
Basically this is about displaying the
health of raid members in form of
clearly arranged bars. VuhDo is
primarily directed to healing classes,
but will make use to almost any other
class. Moreover several healing spells
or other actions can be asserted to
mouse clicks on those bars
(Click-Heal)."
This is a solid replacement for Grid,
Clique, Healbot, what-have-you. Full
raid frames, ability to bind spells
(and Macros!) to key and mouse clicks
on those frames, integration with the
incoming heals libraries so you can
tell when someone is going to be fine
or not, ability to display HoTs
easily, and best of all -- it works
out of the box. Grid can take some
time to set up properly, but VuhDo
just works. (Celairia)
Player versus Player
Gladius "Gladius adds enemy unit frames to arenas for easier
targeting and focusing. It is highly
configurable and you can disable most
features of this addon."
Gladius is every arena player's dream.
Just like VuhDo it will run out of the
box; you can simply drag the window to
where you'd like and enter an arena.
However, if customisation is your
forté then there will be plenty to
keep you busy. In addition to the
usual Blizzard UI features of tracking
health/mana counts and spellcasting,
(albeit in a prettier way,) Gladius
tracks enemy trinket up/downtime and
drinking/first-aiding, as well as
several important buffs and cooldowns.
(Roble)
Miscellanenous
VisualHeal "VisualHeal is a small and simple but powerful tool
for all classes that visually shows
your heals to others and heals
incoming to you from others by means
of two information-packed, yet
intuitive bars."
It shows all incoming heals to my
target (great for watching overheals,
or interrupting heals to maximize raid
healing). (Hhoky)
SmartRes "SmartRes puts bars on your screen that have timers
for the various resurection spells:
Resurrection, Ancestral Spirit,
Revive, Redemption, and tells you who
is casting on whom, how long it will
take to bring the target back to
life."
I also use an incoming Rez addon,
called SmartRes, but that's just my
personal preference, to maximize
rez's. (Hhoky)
Class-specific
PoMTracker "A mod that tracks who Prayer of Mending is
jumping to and how many charges you
have left. Displaying it in a small
movable window frame." (Hhoky)
Best Answer
So there's two real issues here.
How to start near your Friends
The first is a question of start location. In WoW, this is wholly dependent on your race. As of Cataclysm, each race has it's own unique start location, which means that if you pick different races, you will not be 'born together'.
HOWEVER:
It is extremely easy to get to the capital cities, and you reach these at about level 5, after completing your starting zone. And once you're in one capital city, it is very easy to get to any other capital city.
So my advice: if you don't mind playing separately for the first five levels (should take about an hour or so, if you go slowly), play whatever race you want.
All roads lead to Orgrimmar, the Orc capital -- if your friends play Undead, there's a zeppelin in the undead capital to take you to Orgrimmar, and if your friends want to play a Blood Elf, there's transport from the blood elf capital to the undead capitol, and from there you can get to Orgrimmar the same way. Tauren also have a zeppelin (from their capital) to Orgirmmar as well, these days.
Thanks, Powerlord
The Alliance has similar arrangements with Stormwind as the hub.
On the Tank / DPS / Healer trinity
First off, every class can do damage. While you might hear grumblings on the forums that one class or the other is 'better', Blizzard generally does a good job at keeping people even. Never let someone else convince you the class you picked is 'bad'. Blizzard does not make 'bad' classes. But everyone has a personal preference, and invariably these preferences clash, and so you get the "Nerf Wars" chorus, summed up by the pithy:
Damage Dealers
While all classes can deal damage, there are four classes that can exclusively deal damage. That is to say, they cannot tank or heal -- they simply don't have the options. These classes are the following:
Healers
Likewise, there are 4 classes capable of healing, totaling 5 unique subsets of healer. They can cast heals on other players, resurrect dead friends, and otherwise mitigate death and damage. These classs are as following:
Tanks
Finally, there are 4 classes capable of tanking. This means mitigating damage, taunting mobs into hitting you first (and the party later), and other 'defender-y' things.
The Death Knight is a special case, as they are not available to players just starting out. The class becomes available when one of your characters reaches level 55.
Class Conclusions
As you may have noticed, Paladins and Druids showed up on both the "Tank" and "Healer" lists. They are good candidates if you or your friends want to potentially fill any of the roles.
My personal suggestion is that selecting any of the 6 'hybrid' classes is a good choice for starting character, just in case you want to switch roles later. The other thing is that though these classes can spec for these roles starting at level 10, they generally won't be needed while leveling up via quests etc. Three DPS specs will likely progress much faster than a dedicated tank, DPSer, and healer. The exception is if you want to run a dungeon (called an instance) with your pals, where the three roles are strongly encouraged (more so at the highest levels).
However, starting at level 30, your character can purchase the ability to alternate between two separate talent configurations (often referred to as 'dual spec') which means you and your pals can stick as 3 DPSers while questing, and break into your roles when you go dungeon diving. Depending on the exact class, this may require at least carrying around a spare shield in your inventory.
In Conclusion (TLDR)
WoW is a very social game, and any three classes will work well together. Likewise, it is quite easy to meet up, so don't let starting in different areas prevent you from playing what you want.
It's your character. Play what you want. WoW will make it work.
There's little worse than sinking time and money into a character you're only half invested in.