Age of Zombies - Cartoony graphics but it looks good when there are like 100 zombies on the screen. Top down shooter
Modern Combat 3 - good graphics, standard Call of Duty style FPS
Shadowgun - good graphics, heavy influence from Gears of War (3rd person shooter)
Blood & Glory - not a shooter but its pretty and its free
Raging thunder - fun arcade style racer
and i think there are some more flash style racing games on Kongregate