Spells to reflect

Many mobs can have their offensive spells reflected back at them by the Warrior Spell Reflection ability. In a group situation, reflecting such spells this way gives three advantages over merely taking the hit:
 * Mitigates damage that would otherwise be taken, reducing strain on healers
 * Increases the tank's threat, sometimes greatly
 * Inflicts damage on the mob

General Notes
On pulls where you get cast at a lot (such as the packs of four Ritualists in Heroic Shadow Labyrinth) it's a good idea to just hit it everytime it comes off cooldown – something will be reflected, even if you can't be sure exactly what.

Also makes it possible for you tank certain casters that otherwise would present a problem – for example Coilfang Sirens, who tend to fear once they get into melee range.

Mana-Tombs

 * Nexus-Prince Shaffar's Firebolts and Frostbolts
 * Pandemonius's Void Blast

Sethekk Halls

 * You can reflect the Polymorph from Talon King Ikiss. It will not sheep him, but you won't get sheeped, either.
 * You can similarly reflect the Chain Lightning from Darkweaver Syth.
 * You can also reflect Arcane Lightning from Sethekk Oracles. Deals full damage when reflected.
 * Avoid being feared from the Sethekk Prophets by reflecting it (beware of a feared Prophet running around too far and aggroing adds).
 * Reflect Talon of Justice from a Sethekk Talon Lord to stun him for 5 seconds.

Shadow Labyrinth

 * Ambassador Hellmaw's fear
 * Cabal Shadow Priest's Mind Flay
 * Cabal Ritualists various spells can be reflected.

Black Temple

 * Reliquary of Souls, Spell Reflect whenever Deaden is up, starting approximately 30 seconds into the Desire phase.

Battle for Mount Hyjal

 * There are some waves of mobs consisting purely of casters. By running in towards them, they all stop at once and begin casting - you can then hit spell reflect and run away. Doing this reflects about 12 ~4k shadowbolts simultaneously.

Escape from Durnholde Keep

 * The pally boss' HoJ is reflectable (although it does not stun him).

Opening the Dark Portal

 * Aeonus's Time Stop can be reflected. Doesn't freeze him, but it also doesn't freeze you.
 * Pyroblast from the mage Rift Keepers. It does not damage them, but will prevent you from taking damage. You are also able to reflect their blast wave (although Spell Reflect normally does not work with AoE spells).
 * Shadowbolt Volley from the warlock Keepers, it's not frequent so bank the rage. Also, hitting spell reflect when they are first pulled or when they are casting Fear on another party member can cause their +50% damage curse to be reflected, greatly helping any magic DPS classes deal damage to them.

Serpentshrine Cavern

 * Prot tanks pretty much MUST reflect Inner Demon shadow bolts on Leotheras the Blind.
 * The trash around Leotheras that put up the Nature dots on you, if reflected the dot hit for 15k damage on the mob per tick. (~2k ticks on player).
 * Atrophic Blow of the Underbog Colossi. Basically hitting it every time its up will get it burned immediately. You can also reflect the green "spout," basically just stand in its way.
 * Shatter Armor from the Coilfang Shatterers in each of the six packs around the Lurker Below.
 * The Lurker Below's Spout. (Tested Feb 11, 2008 and does NOT reflect...did this person mean "Geyser"?)

Slave Pens

 * Quagmirran – taunt during the Acid Geyser to force him to face you, then Spell Reflect to mitigate damage as well as further DPS the boss. (Does not work in Heroic.)
 * Mennu the Betrayer's Lightning Bolt can be reflected.

Steamvault

 * You can spell reflect the AoE fear from Coilfang Sirens.
 * Shrink Ray and Electrified Net of Mekgineer Steamrigger can be reflected.

Underbog

 * Poison Spit from the Lykul Wasps

Blood Furnace

 * Keli'dan the Breaker's Shadow Bolt Volley – instant, but he casts it right after his Burning Nova most of the time.

Hellfire Ramparts

 * Nazan's Fireballs can be reflected.
 * Bleeding Hollow Scryers Shadow Bolt, very effective due to them being in a group of 5 and often casting at the same time

Shattered Halls

 * Grand Warlock Nethekurse makes "shadow portals" on the floor. Standing in one and reflecting deals 1000 damage per second to the boss for the duration of your spell reflect.
 * Shattered Hand Assassins poisons can strangely enough be reflected.

Magisters' Terrace

 * Vexallus's Energy Bolt. This is an instant cast, so just pop spell reflect whenever it's up.
 * Kael'thas Sunstrider's Fireball.

Karazhan

 * Spell Reflect before Attumen the Huntsman applies his curse, which is triggered every thirty seconds.
 * Skeletal Waiters Brittle Bones is an unclassed debuff, but reflectable.
 * The tank can reflect Maiden of Virtue's Holy Wrath. Also an offtank standing at the edge of Maiden's platform can reflect Holy Fire.
 * Reflect the Ice Tombs on the Skeletal Ushers to prevent being stunned.

The Eye

 * The Overcharge ability from the Crystalcore Devastators can be reflected.
 * The Hammer of Justice from Bloodwarder Vindicators can be reflected, but only you won't get stunned. It is applied almost every 10 seconds so all stuns can be reflected.
 * The Flame Patches from Al'ar can be "cleaned up" with Spell Reflect. The patches takes around 4 ticks to kill, but do not consume the Spell Reflect charge. Be wary of resists and don't continue to stand in the patch if the spell reflect buff expires. (This no longer works as of Patch 2.4.)

The Mechanar

 * Sunseeker Astromage's Solarburn and Scorch.
 * Tempest-Forge Destroyer's Charged Fist. Interrupting this is a better call as Spell Reflect only reflects the lightning damage.

The Botanica

 * Bloodwarder Greenkeepers Poison & Nature Shock can be reflected.

Ring of Blood quest chain

 * Mogor the Ogre in Ring of Blood casts Chain Lightning dealing 4000 damage.