sneak attack feat

[Theodred theOld]

Anyone know the name of the feat that allows sneak attack damage on undead and constructs?

[Crake]

not a feat, but there is an ACF that lets you sneak attack flanked enemies that are normally immune to sneak attack for half damage dice, as long as they can still be flanked, which most undead and constructs can be (though not oozes). Note that it only reduces the damage dice by half, so having craven still allows you to do full craven damage, though the flanking requirement means you'll need to be in melee, or find some way to flank with a ranged weapon.

Alternatively, you can get the weapon augment crystals, the greater demolition crystal gives you 1d6 extra damage, is considered adamantine for overcoming construct DR and lets you sneak attack and crit constructs, and the greater truedeath crystal does the same for undead (but gives ghost strike instead of adamantine vs construct DR). Greater Demolition is 6000gp, and greater truedeath is 10,000gp

[Curmudgeon]

If you're specifically targeting Undead, there's a different Rogue ACF than the one Crake was referring to. Death's Ruin (Complete Champion, page 51) doesn't require flanking, though it only works against Undead.

Additionally, the spell Grave Strike (Spell Compendium) lets you sneak attack Undead for the rest of the round.

[Theodred theOld]

I think it's called penetrating strike. Dungeonscape maybe?

[Curmudgeon]

I think it's called penetrating strike. Dungeonscape maybe?
That's the worst of the various ACFs, as it doesn't grant sneak attack but rather untyped bonus damage.

Edit: Also, still not a feat.

[General Sajaru]

Grave Strike is also in CAd, along with Golem Strike, both of which allow one round of sneak attack on undead and constructs, respectively. I'm not aware of any feat that gives a similar ability though.

[Theodred theOld]

Upon further research I discovered that it is an ACF after all. Not the worst one but also not the best.