Here's a fairly new one: The word "Experience" is used to define VR experiences which doesn't have any gameplay but is more about the experience.

I also found these: (please add more!)
Christian Video Game
Battle Royale Game
HOG
Walking Simulator
MUD
Cute em' Up

Having lived through the MUD/MOO/MUCK/MUSH fad, I don't feel they qualify as videogames.

I once read a book that had a list of games in it by genre, one of the genre's had only 1 game in it and that was

Submarine Simulator (Silent Hunter Series)

Considering i was MUDing back in the mid 80s and we host 3 today on our servers, I wouldn't call them a fad.

There are quite many Submarine Simulators in the world.
And, Submarine Simulator is a sub-genre, Simulator itself is very popular.

:-D

Yeah I don't know if I should laugh or cry at that pun. :P

A few more:

Endless Runner
Play-by-email
Serious Games
One Button Games

Can't think of too many weirder things than sexual harassment simulators, such as this.

http://en.wikipedia.org/wiki/RapeLay

Built for PC games.

Seems every title now is a bad port of a console title.

HOG's are not very obscure, it is probably one of the most prolific PC games genre today.

Same with walking simulators, it is numerous games who belongs in that genre now, usually with elements of horror.

what about 1st Person Rogue-likes / Rouge-lites

I think I'll have to go with Turn Based Flight Simulator, like Over the Reich and its sequel, Achtung Spitfire.

Played a demo of one of these years ago - always wished it would show up here.

I think First Person RTS has to only have a few entries in its listing.

What are you talking about? IBS genre games are wildly popular with titles such as Dr. Diverticulitis, Hemorrhoid Hunter, Hemorrhoid Hunter 2: The Wrath of Crohn, ColoWrecktal, Assteroids and of course the one that started it all Captain Colostomy.