Race
Arkamoi (MMV p.184) -- The LA hurts, but they do make for a fun mailman build.

Item
Cloud Cloak -- Offers a great way to sneak attack.

Belt of the Dread Emperor (BoVD p.114) -- Pricey at 120,000gp, but spell slots are spells slots. A nice gift for an evil Duskblade with Arcane Preparation.

PrC
Ashworm Dragoon (Sandstorm p.66) -- I love the 4th level ability. Combine it with ways to make your enemy attack your mount instead of you, and it's gold.

Feat
Echoing Spell (SoX p.134) -- What's better than being able to cast a spell? Casting it and not forgetting it, of course!

Quick Reconnoiter (CAdv p.112) -- This feat just needs more love. I will state, here and now, that this feat should be as important to someone focusing on Spot and Listen as Darkstalker is to someone focusing on Hide and Move Silently.

Spell
Aberrate (BoVD p.84) -- Combine with Aberration Bane Magic feat and make all of your enemies susceptible. A combo used in my A Bump In The Night entry for Junkyard Wars VII.

Misc
Necrosis Carnex (MMIV p.104) -- Creatable with an Animate Dead spell, animal intelligence, necrotic touch that heals undead, and a debuff aura (that is, incidentally, untyped). No necromaster should be without these.

Magical Storage Companion Spirit (DMGII p.196) -- Cast a spell and grant all of your friends access to using it, including you getting your spell back! If you think that wands are useful, this is for you.