Im Bored Im looking to buy a  cheap 20$ or less game since im low on funds and video games are never that big of a priority on me.
 
so go ahead what are your favorite games that are 20$  or less.

Mass Effect is my favorite game of all time, and it's around 20 or less. Dead Space is good, Mirror's Edge can be found for around 10, man just go to your Blockbuster or Gamestop and look at the used section. There are so many. 

Pinball Hall of Fame The Williams Collection.  It's $10 in store at Gamestop (actually it was $20 just a week or 2 ago).  It's an excellent game and it looks like Gamestop wants to flush their stock.

Shadow Complex.

Too Human, Mass Effect, Ninja Gaiden 2, Street Fighter 4, Battlefield Bad Company

Elder Scrolls IV: Oblivion. You can get that game ridiculously cheap nowadays, and a single playthrough will keep you delighted for 120 hours. 
 
@EpicSteve said:

" A gamefly subscription. Then you play everything! :) "

A gamefly subscription. Then you play everything! :)

If you like first-person RPG's, then Fallout 3 and Oblivion are likely your best values for $20.  You can easily spend 100 hours playing them and still not see everything.

Banjo-Kazooie:Nuts&Bolts

"Banjo-Kazooie:Nuts&Bolts "

Banjo, Dead Space, or Mass Effect.

Just Cause if you can actually find it anywhere.
Gun (probably under $10)
Burnout Paradise 
Viva Pinata
 
are the first one's that come to mind for me.

Arcade: BF1943, Trials HD, Castle Crashers, Geometry wars 1/2 
Full Games: Halo 1 and 2, Orange Box, BF 2, Bad Company 1, Ninja Gaiden 2, etc.

Ninja Gaiden 2.

 Dude, there's a ton. Really. Pretty much every good 360 game that wasn't released within the last year should be down to $20 by now. Here's some good ones:
 
Banjo-Kazooie: Nuts & Bolts, The Orange Box, The Darkness, FEAR, FEAR 2, Dead Space, Ninja Gaiden 2, Kameo, Skate, BioShock, Tony Hawk's Project 8, Burnout Paradise, The Chronicles of Riddick: Assault on Dark Athena, Fable 2, Red Faction Guerilla ($20 on Amazon right now), Battlefield: Bad Company, Crackdown, Mass Effect, Saints Row 2 (Though I'd advise you to stay away from Saints Row 1 at this point), Condemned: Criminal Origins, Condemned 2: Bloodshot, Halo Wars (Also on sale for $20 on Amazon now), Prince of Persia... and man, there's more if you just do a little looking around. Plus there's a lot of great downloadable games over Xbox Live, and I think all of them are under $20.

Orange Box, Oblivion, Dead Space (Games on Demand), Burnout Paradise (Games on Demand.)
 
Those will last you a while except for Dead Space, which'll be around 10-12 hours the first time you play. But if you want to S-Rank it you'll probably do two more playthroughs.