The Argent Crusade
The Argent Crusade faction consists of Paladins that made their home on Northrend. The main hub of The Argent Crusade is in Icecrown, right near the border of Crystalsong Forest. You can raise The Argent Crusade faction one of two ways, do the dailies in Icecrown or wear the Argent Crusade tabard while running a level 80 instance. Since level 80 instances are much easier now, I can solidly recommend farming Reputation that way.
I prefer Halls of Stone (I only clear the trash and easy bosses, I stop at the escort) or Halls of Lightning. The Argent Crusade doesn't have many dailies available to them that are worth doing. So this would be a wise faction to wear the Tabard for.
The Argent Crusade faction can be raised by doing dailies and wearing their tabard in instances. Very similar to the Shattered Sun Offensive reputation you can also raise your Argent Crusade reputation with gold. For a link to the quest you can raise your reputation with via gold click here. Below is a list of the only Argent Crusade daily worth doing (In my opinion)
Slaves to Saronite
You can get this quest from Absalan the Pious aboard the Skybreaker for Alliance and Brother Keltan aboard Ogrim's Hammer for Horde. For this quest you are required to head over to Ymirheim and enter the large ass cave. In here you will need to talk to the miners until 10 of them are freed.
Argent Crusade Reputation Achievements
The Argent Dawn: Earn Exalted Status with the Argent Dawn. Pretty easy, just run a lot of Scholo and Strath.
The Argent Champion: Raise your reputation to exalted with The Argent Crusade and The Argent Dawn. The Argent Dawn is a classic WoW faction that used to be a max level (level 60) faction but with the revamp Cataclysm brought to the world Eastern Plaguelands is now a 40s zone. Also Argent Dawn Reputation can no longer be gained from running Strath or Scholo you now must complete quests in Eastern Plaguelands to raise your reputation. As for the Argent Crusade they are still a Northrend faction primarily found in Icecrown.