A guy on YouTube named Mikey Bolts just put up a cover of Pharrell Williams' "Happy" sung in different voices from "Family Guy".  And all the impressions are pretty dead-on.  Although if you're sick of the song "Happy" and you hate "Family Guy" ... this is pretty much your worst nightmare. 

He sings the lead vocal as Stewie, and the chorus as Peter.  And once in a while he throws in Herbert, the old perverted dude who whistles when he talks.

FCC WARNING: This clip includes the word "farts."  Play it at your own risk.

There's a guy named Roomie on YouTube who just posted a video of himself singing 14 different famous songs, all in the voices of the original artists.  Some of them are really good.  

This clip has his impressions of Adam Levine from Maroon 5, Jon Bon Jovi, Rick Astley, Jason Derulo, Gary LeVox from Rascal Flatts, Dan Reynolds from Imagine Dragons, and Eduard Khil, the "Trololo" guy. 

WARNING: the video has an uncensored F-bomb.