Shadow resistance

As stated in the Resistance article, resistance gear should only be used when it's really needed. There are various sources for shadow resistance (like green random drop items and pre-BC sets crafted from Dark Runes), but it's not advisable to use these items at level 70, because they're too weak and shadow resistance is generally of little use.

Useful in...
In the following instances, shadow resistance can be useful:
 * Scholomance and Stratholme: shadow resistance can make these instances easier, but it's by no means required (even the priest buff is not really needed)
 * Blackwing Lair: The tanks on Ebonroc need shadow resistance
 * Temple of Ahn'Qiraj: essential for Warlock Tanks at the Twin Emperors
 * Mount Hyjal: on some bosses medium amounts of shadow resistance are highly recommended
 * Black Temple: high amounts of shadow resistance required at Mother Shahraz, medium to low levels in the entire instance

PvP
Besides that, shadow resistance can also be useful in PvP when fighting shadow Priests and Warlocks.

Nonetheless, Resilience is generally considered to be the more useful stat in PvP.

Mount Hyjal and Black Temple
Mount Hyjal and Black Temple are the only places which cannot be mastered without shadow resistance, and incidentally these are the only places where Heart of Darkness drops. Therefore, raids entering Mount Hyjal start to find these items, and are well advised to save them and set up a policy how to craft shadow resistance gear for everybody, because at Mother Shahraz the whole raid really needs it.

The first good shadow resistance item is gained by the Black Temple Attunement, it's the Medallion of Karabor. The next item everybody should get made is Night's End which needs only one Heart of Darkness to craft. The following table lists the remaining items together with the number of HoDs needed, and the amount of resistance gained for each HoD used.

Mother Shahraz
The only encounter in the entire game which needs really large amounts of shadow resistance is Mother Shahraz. Assuming the raid is buffed with Rank 2 Prayer of Shadow Protection and everyone has a Medallion of Karabor, the resistance cap of 365 can be reached with the 5 pieces of crafted gear and no shadow resistance enchantments at all. Since equipping 24 raid members with the full set would require about (8 * 30 =) 240 HoD, many raids won't reach this goal. Most frequently, some raid members miss the Legs. In these cases, some enchants should be used and/or a few gems or green items to reach the cap. Reaching the cap is not a requirement for beating this encounter, especially once the players are better at moving away from one another during Fatal Attraction.

Resistance addition
Full crafted gear:
 * 70 - Rank 2 Prayer of Shadow Protection
 * 40 - Medallion of Karabor
 * 40 - Night's End
 * 40 - Bracers
 * 54 - Belt
 * 54 - Boots
 * 72 - Legs
 * = 370 Shadow Resistance, which is even 5 over the cap. This set can use the standard enchants.

Omitting the legs and replacing them with a Enchant Cloak - Greater Shadow Resistance on the cloak and a Shadow Armor Kit on the boots:
 * 70 - Rank 2 Prayer of Shadow Protection
 * 40 - Medallion of Karabor
 * 55 - Night's End
 * 40 - Bracers
 * 54 - Belt
 * 62 - Boots
 * = 321 Shadow Resistance

Omitting the Medallion of Karabor or Bracers and replacing them with +20SR glyph on helm:
 * 70 - Rank 2 Prayer of Shadow Protection
 * 55 - Night's End
 * 40 - Bracers
 * 54 - Belt
 * 54 - Boots
 * 72 - Legs
 * 20 - Helm with Arcanum of Shadow Warding
 * = 365 Shadow Resistance

Resistance gear by type
Materials needed:
 * 1xHeart of Darkness
 * 1xVoid Crystal
 * 2xPrimal Life
 * 2xPrimal Shadow
 * 1xBolt of Imbued Netherweave

Materials needed:
 * 8xHeart of Darkness
 * 4xVoid Crystal
 * 11xPrimal Life
 * 16xPrimal Shadow
 * 5xBolt of Imbued Netherweave

Materials needed:
 * 8xHeart of Darkness
 * 4xVoid Crystal
 * 11xPrimal Life
 * 16xPrimal Shadow
 * 11xFel Hide

Materials needed:
 * 8xHeart of Darkness
 * 4xVoid Crystal
 * 11xPrimal Life
 * 16xPrimal Shadow
 * 11xFel Scales

Materials needed:
 * 8xHeart of Darkness
 * 4xVoid Crystal
 * 11xPrimal Life
 * 16xPrimal Shadow
 * 28xAdamantite Bar

Materials needed for gems:
 * 4xLarge Prismatic Shard
 * 2xVoid Crystal