My favorite aberrant monster in D&D is most definitely the illithid. Also known as the mind flayer, they're some of the most interesting Big Bad Guy monsters out there. They're almost always ridiculously intelligent, they've got their mind blast and tough psionic powers, and they're generally surrounded by their thrall minions. On top of all that, they eat their victims' brains.
From the Far Realm (or your campaign setting's equivalent), mind flayers are utterly alien and irrevocably evil. With their intelligence and penchant for enthralling some of the tougher victims they catch, they make great boss-type enemies. They're perfect for dungeon crawl adventures; given that they're intelligent and often psions or wizards, they make for a great way to combine the classic wizard's laboratory and a standard cave-style dungeon. They're great for Underdark adventures, but with all of their history, they're an even better enemy for multi-planar adventures, especially if the githyanki or githzerai are involved.
On a different note, I've been trying to put together a big blog post detailing some setting stuff I've been working on, a few ideas on implementing new-school races like warforged or dragonborn in old-school games like Swords & Wizardry and Dark Dungeons, and maybe a few adventure hooks and ideas. I'm hoping to get it up sometime this week, but I'm not going to make any definite statements yet. Once this D&D 30 Day Challenge is finished up, I plan on making at least two posts a week, but for the most part, they'll be longer than these challenge posts.