Shifting Naaru Sliver is an epic trinket for DPS casters.
On top of the quite substantial 54 haste rating already offered by this trinket, it has a very powerful on-use effect, which essentially grants an additional:
320 * (15 / 90) = 53.3 +damage
In stationary fights, provided it is activated upon every cooldown.
Patch 2.4.2 (2008-05-13): Cooldown lowered from 2 minutes to 1.5 minutes.
Patch 2.4.2 (2008-05-13): Using this item no longer puts players into combat.
Patch 2.4.2 (2008-05-13): This item now only benefits the caster.