Serra Avatar 4WWW, Urza's Saga, Rare.
Summon Avatar
Serra Avatar's power and toughness are each equal to your life total. 
If Serra Avatar would be put into a graveyard from anywhere, shuffle Serra Avatar into its owner's library instead.
Well, everyone probably has heard of this card, and its power.  Yet, the main problem lies in its cost of 7 converted mana.  Even if you play with mana elves, birds, or some other source of mana acceleration, you'll be hard pressed to get this baby out at its 20/20 power level (unless you're a lucky stiff that got an Elvish Piper out on round 3 and have an Avatar in your hand).
Anyway, I've created a deck that not only gets the avatar out fairly early, but also gives large amounts of life for yourself, thus giving the avatar lots of power and toughness (largest I've seen was 1000+/1000+, but it could have been a lot higher if I didn't quit the game).
To get it out:
Oath of Druids Exodus Rare 1G
At the beginning of each player's upkeep, if that player controls fewer creatures than an opponent, the player may reveal cards from the top of his or her library until he or she reveals a creature card.  The player puts that creature into play and all other cards revealed this way into his or her graveyard. 
Now, basically, you just wait until an opponent gets a creature out, and if Serra Avatar is the only creature in your deck, then you're guaranteed to get it.  However, to minimize the number of cards you're losing to the graveyard, use Worldly Tutors or Gaea's Blessings.  Alternatively, you can use this:
Defense of the Heart Urza's Legacy Rare 3G
At the beginning of your upkeep, if an opponent controls three or more creatures, sacrifice Defense of the Heart to search your library for up to two creature cards and put those creatures into play.  Then shuffle your library.
Now, granted, this is more expensive and you'll probably suffer some damage from three creatures, but you won't be losing cards in your library, or use Tutors and Blessings.  And you'll also get 2 Avatars, instead of the 1 from Oath of Druids.
For life:
Noble Purpose, Spirit Link, some other cards that basically do the same thing:
Whenever a creature you control deals damage, you gain that much life.
Angelic Chorus Urza's Saga Rare 3WW
Whenever a creature comes into play under your control, you gain life equal to its toughness.
Again, these are pretty self-explanatory.  Use Noble Purpose/Spirit Link to double your life each time your avatars attack/defend, and whenever you get an avatar, you double your life.  This makes your life gain exponential!
And if 1000/1000 serra avatar's aren't enough to scare your opponent, you can always finish him/her off with Celestial Convergence or give trample to those avatars.