"Youse" is commonly used in Dublin as the plural of you. In other parts of Ireland "ye" is more often used.

Most other languages have both a singular and plural form of "you", but English doesn't. Maybe it should.

It's not a part of "standard English", but it occurs in "non-standard dialects". English teachers generally try their best to stamp out any dialects. Trust me on this, I'm a a linguist. =P

 I am somewhat sometimes too though I make mistakes as everyone does, but I have several dictionaries right here next to me right now as I verified they all have "youse." As mentioned, it may be more of a dialect at least now. I can't say about if it used to be more common. I also know a couple of people from Pennsylvania who use, "yinz" as a plural of you, so I guess if you consider it to be a legitimate word if it is in the dictionary, then "youse" is one. Then again newer dictionaries also have "google" as a verb, so take it as you will. The phasing out was about how they usually try to decrease the use of non-standard and dialect words.

Most of youse agree.

Some of you don't agree :P

You might be a cunning linguist ... but I'm a master debator. 

Booooooh. That joke is old.

Yes ... well ... somebody had to say it. And someday, I wanna be somebody!

It's actually used less by rappers and more by 'guidos' or italian americans. I live around a lot of them in the NY/NJ area so I hear it a lot. The common phrase is 'hey, yous guys" that everyone knows. Well people here actually say it and just like y'all has entered into the dictionary, so has you's [unfortunately].

Dialects are part of common life though and language is always evolving so I don't have any qualms with it. I just wish our language would evolve from words used by smart people [this coming from a half italian]. Guidos are retarded. In case you don't know what that is or you need proof...

http://user10337.websitewizard.com/images/guidos_small.jpg