Here's six off the top of my head that I've had a LOT of fun with, as either a player or a GM:
Brainstealer Dragon (Dragon Magazine; don't have my books at hand so I'm not sure of the issue #)
Imagine a mind flayer; now imagine a dragon; now imagine a draconic mind flayer. It's a lot like that. Only much, much worse. A great monster, but one to be very careful with because it seems under-CR'd even for a dragon. Its mind blast alone can easily translate into a stun lock followed by a TPK.
Deathjack (Monsternomicon)
It's a steam-powered mecha, right? How bad can it be? Well, it's a steam powered mecha that FUELS ITS OWN BOILER WITH THE STILL LIVING BODIES OF ITS ENERGY-DRAINED ENEMIES. That's how bad it can be. A brutal beater with spell-like abilities that actually has a miniature made for it.
Golem, Paper (Dragon Magazine)
A lesser golem made of origami. Great for a low-level antagonist in an Oriental Adventures game, but suitable for other settings.
Golem, Squiggler (Monster Encyclopaedia v. 2)
It's living graffiti, basically. A very fun lesser golem that's not overwhelmingly powerful but CAN be overwhelmingly creepy.
Grass Devil (Monster Encyclopaedia v. 2)
A vicious little menace that can entangle and, in large numbers, control plants. It doesn't have any kind of a finishing move and is easy to kill, but oh is it annoying to the PCs! Also, very dangerous when combined with another monster (especially a flyer). Try throwing a wyvern or equivalent at even 7th or 8th level PCs who are dealing with a field inhabited grass devils, and I guarantee it will be a memorable encounter.
Thornwood Mauler (Monsternomicon)
Your druids will love you for it - if they survive. Think of this powerful animal as a cross between a prehistoric carnivorous giant sloth and a Games Workshop dark eldar who has been overzealously covered with spiky bits. Its not smart, it largely wants to be left alone, and it WILL kill every last adventurer in the room if not stopped. Of course, a 12th level druid who has met one can then turn into it - can, and probably will.