Are there any items that grant sneak attack?

[Rezialn]

Theory crafting sneak attack builds with a friend. If anyone knows of any items, it'd be appreciated.

[Nibbens]

The Mantis Blade (http://www.d20pfsrd.com/magic-items/artifacts/minor-artifacts/mantis-blade) from PF... But that's a minor artifact.

Also, the Ring of Swarming Stabs (http://www.d20pfsrd.com/magic-items/rings/ring-of-swarming-stabs). Also from PF... although that just "counts as a sneak attack" for limited purposes - but it might fall into vein with what you're looking for.

[Jormengand]

Assuming you already have sneak attack from somewhere... (http://www.d20srd.org/srd/epic/magicItems/weapons.htm#backstabber)

[Florian]

Theory crafting sneak attack builds with a friend. If anyone knows of any items, it'd be appreciated.

You mean an item that gives the Sneak Attack class feature or an item that upgrades an existing Sneak Attack class feature?

Besides some Artifacts, not really nothing there, sorry.

[Telonius]

Possibly a scroll of Nightstalker's Transformation (SpC). It doesn't grant it as a class feature, but it allows you to sneak attack for the duration of the spell. (EDIT: didn't see pathfinder in the title; that's 3.5).

[CrazyNoob]

Ranger Lvl 2 Spell Hunter's Eye (PHB2 pg115), grants you 1d6 Sneak attack / 3 CL
Twilight Knife (PF Spell (http://www.d20pfsrd.com/magic/all-spells/t/twilight-knife))