Rockstar has posted details on the first DLC for Red Dead Redemption titled "Outlaws to the End". The DLC, which will be released in June, is free and aims to add some co-op action to Red Dead Redemption's multiplayer. There will be six missions available in the DLC, three of which have been detailed:

“Walton’s Gold”
Walton’s Gang have taken control of a mining camp rich with gold.  Fight through the camp and load your mine cart with as much gold as you can carry and get out as fast as you can – Walton’s boys have rigged the place to blow!

“The River”
Ride a raft down river, taking out rebel encampments along the way until you reach the rebel stronghold of Nosalida and a final epic battle for the town’s massive weapons caches. Watch out for Gatling gun-equipped enemy rafts.

“Ammunition”
The Mexican Army has the town of Tesoro Azul under siege.  Storm the gates under heavy cannon, Gatling gun and sniper fire to destroy the Mexican artillery placements.

The DLC will also add some XP unlocks and new achievements/trophies. This is just the start of DLC for Red Dead Redemption as Rockstar plans to release two more paid DLC packs sometime after the release of this DLC.