I mean, they're somewhat known, but Monopoly and You Don't Know Jack. Dating someone who has the WORST reflexes ever we're forced to play games that require thought and not reflex. Monopoly needs no introduction and YDKJ is a gameshow type ordeal that has customized announcers and stuff. It's hilarious. You need to watch a video of some gameplay.

Also, for Xbox Live Arcade, Marble Blast Ultra. It's an arcade game that I think is only $5 now and I've easily put more hours into it than most of my other 360 games. Also, Costume Quest. It's a cute little RPG that'll have you laughing and is definitely worth the money. And N+ on the marketplace. It's an action platforming game with hundreds of small levels with a ninja.

1 vs. 100 Oh how I miss that game.

'Splosion Man in the XBLA.
I think only one other person on my friends list has it but this game is a ton of fun. It's a platformer puzzle game.

The Ghostbusters video game was way underrated. It is one of my favorites, I still throw it in from time to time.
Written by and staring the originals, except Rick Moranis, it's the story of what would have been the third movie had it been made.

@wickedd365: I agree, 1vs100 was a good time! By emailing a trivia answer during the show, I was able to win a year's subscription to Last FM. Also, they called me back once to give a shout-out to my brother live on the air! I never did get to be the "one" or even in the mob, but I got all of my achievements!
'Splosion Man was fun for a while, but I don't think I have the patience for it... I kept hitting spots where I died a lot and had to start over. I know you can skip the level after a while, but then it makes you wear that degrading tutu. Thanks for reminding me of Ghostbusters, I played the demo, but never bought it. I reckon I can find it used for cheap now.

EDIT: Also, I'd vote your comments up if I code, but when I try woot claims I haven't bought anything! Lies!

It's not wildly underground, but I would play I <3 Katamari over and over.

@rhbama13: I MAED A GAM3 W1TH Z0MBIES 1N IT!!!1

For some reason I just dont believe you were capable of managing this...

I also feel its just not worth my time/effort to go through them all...

Costume Quest was super fun.

Halo 1

Not really unknown, but very underrated in my opinion: Viva Piñata. It's like Pokemon meets Harvest Moon. Very addicting and fun game, crazy cheap these days :)

The whole Indie Arcade catalog is golden.

I've played pretty much every XBL Indie game since before there was an Indie section heh... Back when they were part of the "XNA Creators Club" and you needed an unstable installer to play the user-created games.

Little known fact? "The Dishwasher" (an Xbox LIVE! Arcade game) started out as an XNA Created game. Dunno what happened, but Microsoft bought it into the fold.

^ Proof that we will never see any 5-Star, Triple-A user-created titles in the Indie section - Microsoft will snatch up all of those games and sell 'em in the regular XBLA for 1200MS.

[edit]

The Dishwasher when it was a baby:
http://www.youtube.com/watch?v=THELa9pO23E
(I also miss 1 v 100..never won anything though.)

chime is fantastic if you like music games.

lost odyssey is a very good RPG that i am actually playing through (again).

if you like card/board type games you might want to look into carcasonne and catan, both are very good and based fairly closely on the board games from what my friend tells me (ive never played the board games).

if you like shooters id recommend looking into rainbow six: vegas 2, its been out for ages and is pretty cheap preowned but still fun to play.

I thought they were able to figure out the filter thing over here because I MAED A GAM3 W1TH Z0MBIES 1N IT!!!1 seemed too ridiculous. I'm going to have to download that one.

The Sabotuer. It's like a mix of Assassin's Creed and GTA, and set in Paris. One of the few games I still wanted to play after beating it.

TRIALS HD

Gotta try the beautiful, totally fun Trials HD. LOVE IT. So easy to pick up and play.

SHADOW COMPLEX

Shadow Complex just blows me away. It's a side scroller, sorta... Platformer... Spy Game with incredibly unique gameplay, style and graphics. It's just COOL. It's another game that's very easy to pick up and play and you can jam through it, or take your time and figure out all the puzzles. Try it. It's incredible.

@rhbama13: I got the zombie game. The soundtrack is both funny and good. Sadly, like most indie games, there are no achievements. There is also no online leaderboard to compare my score with my friends. Definitely no online play. Very short game, but good for a change of pace. Not a whole lot of replay value.

Psychonauts. I know it's for XBOX (After the people who owned the game complained loud enough for long enough, it was eventually made compatible with the 360), but it's probably the best little-known game out there. A lot of the game is spent literally inside of crazy peoples' minds...

A review by a guy with a british accent:
http://www.escapistmagazine.com/videos/view/zero-punctuation/2-Psychonauts

Monday Night Combat.

Class based team third person shooter with a tower defense aspect. Kinda like TF2 meets League of Legends. Came out a bit over a year ago and still have a pretty big community for an XBLA game. You can get a free 1 hour demo if you want to check it out, don't get frustrated though, it does have a bit of a learning curve.

Thanks for the responses, all. Keep 'em coming! So far, from the suggestions, I've only purchased "I made a game with zombies in it" and "You don't know Jack". I'm trying to at least play the demos for the games mentioned here.

@jbilde I wasn't too impressed with the demo for "Too Human" when it first came out. I suppose I can find it for cheap nowadays.

I'm currently playing Alan Wake. Its off the shelf, but had the misfortune of being released the same day as Red Dead Remption. Alan Wake could be a full franchise game for the 360, but Microsoft isn't that smart.