Argent Dawn

Some say that it is always darkest just before the dawn. Whenever darkness falls, the light of hope will drive away the shadows; wherever the forces of evil work in secret, the brave men and women of the Argent Dawn will be there to thwart their dark designs. The Argent Dawn is an organization that makes no distinction between Alliance or Horde, but instead tries to unite the people of Azeroth in the fight against evil. Though the agents of the Argent Dawn are always wary of the sinister and the wicked in all their various disguises, they have recently focused their efforts on a region of Azeroth where evil has run unchecked for a long time: the Plaguelands of Lordaeron. Detachments of the Argent Dawn are guarding the entrances to the Plaguelands at the Bulwark between the Tirisfal Glades and the Western Plaguelands, and at Chillwind Point, a narrow pass leading south from the Western Plaguelands to the Alterac Mountains. They have also gained a foothold deep in the Eastern Plaguelands at the Light's Hope Chapel, east of Corin's Crossing.
The Argent Dawn is always looking for more fighters to join their cause, and the numbers of those wearing the Argent Dawn Commission, the organization's insignia, are growing each day. Adventurers seeking to aid the Argent Dawn by rooting out the Scourge's corruption in the Plaguelands and taking on the Lich King's powerful servants in the Scholomance and Stratholme will be well rewarded; the organization has many powerful enchanters among their ranks who craft special enhancements that provide the wearer with added protection against all forms of magic attacks. The Argent Dawn also offers those who have proven their dedication to their cause insight into their craftsmen's designs, enabling these adventurers to craft the armors the Argent Dawn created in their ongoing campaign against evil.

Tips

Working your way up in Argent Dawn reputation will unlock recipes, rewards, and it will also be very important to enter Naxxramas.
Argent Dawn Commission
After reaching level 50, go pick up an Argent Dawn Commission. This is a trinket that allows you to loot Scourge stones from Undead monsters in Western and Eastern Plaguelands, Scholomance, and Stratholme. You can also gain Scourge stones from the Scourge Invasion while that event is active. Your trinket must be equipped in order to see and loot Scourge stones. If another player doesn't have their trinket on, you can take any Scourge stones that would have dropped for them while you're in the same group as that player.

Alliance players can get their Argent Dawn Commission from Chillwind Camp in Western Plaguelands. Horde players can pick theirs up at Bulwark in Tirisfal Glades.

You can also, through a series of quests, get a Seal of the Dawn or Rune of the Dawn that replaces the Argent Dawn commission and also gives additional benefits to killing undead.

Scourge stones
Scourge stones can be turned in via repeatable quests for reputation. They cannot be traded.

Scourge Stones come in three versions:

Minion: Requires 20 for turn-in, found on monsters about level 50 and up
Invader: Requires 10 for turn-in, found on level 53 monsters and up
Corruptor: Requires 1 for turn-in, usually found on bosses
Scourge stones stack in piles of 250. Once the Scourge stones are turned in, you receive an Argent Dawn Valor Token that can be activated to gain 25 reputation. Argent Dawn Valor Tokens stack up to 500. It's recommended to hold on to your Scourge stones until you're Honored with the Argent Dawn. At that point, you can no longer gain reputation by slaying most undead monsters.
Gaining Reputation
You can gain reputation by killing monsters, and completing quests.

Kill Monsters

You can kill regular undead monsters in Western and Eastern Plaguelands until the end of Friendly reputation. You can also kill Wizards of the Cult of the Damned in Eastern Plaguelands.
Next, you can kill the elite undead in Stratholme and Scholomance until the end of Honored reputation. These can also drop Scourge stones.
After that point, you only gain reputation for slaying the bosses in those two instances. You gain additional reputation if you kill Baron Rivendare in Stratholme as part of the 45 minute Baron Run. After you fill up Revered reputation, those bosses no longer give reputation gains.
Again, the Scourge Invasion is a great way to earn reputation via monster kills and Invader's Scourgestones. Regular scourge troops give 5 reputation per kill until Revered and there is a LOT of them. Don't miss out on this event.
Quests
Complete all of the quests you can at Chillwind Camp, Light's Hope Chapel, and any other quests you can find for Scholomance, Stratholme, Western and Eastern Plaguelands. Some recommend waiting until Honored reputation before completing the quests when you can't gain reputation from most undead monsters.
There are some repeatable quests you can do to continue to gain reputation.

Healthy Dragon Scales drop in Scholomance which can be turned in one at a time for 50 reputation. You can take one scale at a time per quest. The drop rate of this scale is somewhat low.
You can also continue to turn in Scourge stones for reputation until Exalted.
There are four Cauldron quests in Western Plaguelands that can be repeated to gain 15-25 reputation
Light's Hope Chapel has several repeatable quests to gather a stack of 30 items. These items can be traded or bought on the auction house. When you first complete the quest you gain 200 reputation. If you complete them again, you only gain 10 reputation, making it less than ideal for long term gains.
Craftsman's Writs - If you turn in an Argent Dawn Valor Token you can get a Craftsman's Writ from Commander Metz. These require you to gather materials or items. You can purchase those items from the auction house, but that can be very costly. You will gain 75 reputation per turn-in.