Creatures that give your other creatures a bonus in power while they are in play are not all that rare or difficult to find, but creatures that give your other creatures a permanent bonus are a little more difficult, and those that can give everything multiple +1/+1 counters are not only rare but very powerful.
Vigor is a rare elemental incarnation from the lorwyn expansion set of magic the gathering. It costs three green and three colorless to put into play and is a 6/6 creature with trample, but the truth is that the size of Vigor is unimportant next to the ability that it gives.
This ability reads "If damage would be dealt to a creature you control other than Vigor, prevent that damage. Put a +1/+1 counter on that creature for each 1 damage prevented this way."
This not only makes every creature in your deck immune to most forms of creature combat and direct damage but it makes them far more powerful when they do it. This is valuable in any deck but even more so in a deck with a great many small creatures, and in green that generally means elves.
The problem with Vigor is of course that your opponent is not going to do damage to your other creatures unless he or she has no other choice. Instead everything is going to be focused on killing Vigor, which most decks should be able to do. This is why it is important if you want to get real value out of Vigor that you play it in a deck that can take advantage of it.
The most obvious of these colors is red. Red has a number of powerful spells which are able to do damage to all creatures. Pyroclasm has long been one of my favorite but you typically take out at least one of your own creatures with it. In the case of vigor though playing it with ten 1/1 creatures out only means that you now have 10 3/3 creatures out rather than ten creatures in the graveyard. This becomes even more impressive if you have a card like earthquake, or simply using lightning bolts as a permanent giant growth.
If you can keep vigor alive for even a turn or two it should be a game winning card and it has to be a first pick in draft where player are less likely to have good removal to kill it.