Affably Evil/Tabletop Games

Nurgle, the Chaos god of disease, decay, despair, and destruction in the Warhammer Fantasy Battle and Warhammer 40,000 settings is a jovial, avuncular figure who lavishes attention on his "children" and sees himself as a creative force in the universe. He is often referred to as Grandfather Nurgle, or even Papa Nurgle. The same can be said for his mortal followers. In the fluff, you'll almost always see them joking or having a good time. At one point, after a POW gave up and agreed to worship Nurgle, his jailer just smiled and hugged him.
The bread and butter of both incarnations of the World Of Darkness.
- This is especially common in the Vampire: The Requiem gameline. Since vampires are evil within the context of mundane human in-game morality, they do what they can to prevent themselves from appearing evil. The Ventrue and the Daeva clans, particularly, thrive on this trope, even going so far as to have an active rivalry between each other as to which style of manipulation (Honey vs. Vinegar) is superior. Unlike their Masquerade counterparts, the Nosferatu of Requiem can also handily play within this trope, and can even outdo their classier and prettier opponents.

In the Mekhet clanbook, Frances is funny, sweet, apologetic, and into indie music. She also brainwashes a house full of female students so she can feed on them, turns her boyfriend into her devoted ghoul ("I think I've broken him"), and toys with a female executive before killing her in the most humiliating way possible.

- Then there's the Tzimisce clan from Vampire: The Masquerade, a race of complete monsters, even by vampire standards, who are generally affable towards guests in their domain. This is due to their ancient hospitality rites, which transcend even their desire to turn everything fleshy into living, tortured abominations. A perfect example is Andrei, a Fiend from Vampire: The Masquerade Bloodlines, who openly, calmly, and truthfully converses with the Player (unless they're Tremere) when they first meet in his mansion decorated by human flesh...
Dungeons and Dragons:
- Archduke Dispater, Lord of the Second Layer of Hell is either this or Faux Affably Evil, depending on who's describing him. He is known to be tyrant, a chessmaster, and a Manipulative Bastard (like most powerful devils) but also a calm, mellow type who never loses his temper, unlike his ally Mephistopheles who has absolutely no control over his. Although, at least one source claims that if he did ever lose his temper, he'd likely annihilate any witnesses to it. One example from the module Fires of Dis demonstrates it:

Dispater: You know, I must confess I'm a bit surprised. I expected something of a ground invasion force — thousands of noble paladins descending upon my hateful plane in the name of all that's holy and so forth. Instead I find you: a band of — you'll pardon the expression — ordinary thrillseekers.

Graz'zt is also known to be civil and sociable, at least when compared to other demons; so long as you never bring up that whole thing about him and Iggwilv, it's kind of a sensitive issue for him...

Forgotten Realms has Sshamath -- technically, it's an evil drow magocracy, and they didn't disown Lolth, having only thrown off the theocracy. But for the sake of trade, it was made as pleasant as a drow city could be. This includes restrictions on slavery and having one of the best taverns catering to any sentient being (with special floors for freshwater, saltwater, and flame-dwelling clients).
A few of the more sociable Darklords in Ravenloft comes off as this. In some campaigns, Strahd Von Zarovich can be downright charming, as long as you're not getting in his way in his never-ending quest to claim the woman he lusted after, or do anything he perceives as harming his beloved Homeland. Harkon Lucas is the classic Bard, a ladies' man who loves to entertain his guests -- it just so happens that he's a man-eating wolfwere, etc.
Pathfinder: Laori Vaus. Dear Zon-Kuthon, Laori Vaus. She's a friendly, upbeat allied NPC cleric in the Curse of the Crimson Throne module, who happens to be a devout follower of the god of darkness, pain, and loss (and not in a Dark Is Not Evil way, either). She's one of the more popular Pathfinder NPCs.
Some of the Krynnish evil deities can be very friendly when they want to be. Sargonnas really does care about his followers, and has standards and a sense of honor. Nuitari can get along well with Solinari and Lunitari, due to the three deities representing Evil, Good, and Neutral magic. He also can get along fairly well with mortals. Chemosh, the God of Undead, is apparently capable of caring enough for another being that he can feel genuine grief when that being is hurt. Zeboim, the Goddess of Storms, really does care for her son. In terms of mortals, the Black Robes frequently come across as being rather nice (Dalamar and Iolanthe) and are able to get along with their White and Red Robed fellow Wizards.
Magic: The Gathering: Even in his Eldritch Abomination form, Yawgmoth keeps his smart, cultured, charismatic personality. He also rewards his faithful underlings with various augmentations, which is seen as one of the highest honors in Phyrexian philosophy.